So a stock priced at 100 doller-pounds and a yield of five percent could be expected to hand a stockholder 5 doller-pounds a year.
Obviously, this is an instantaneous snapshot, based on the price now and the dividend then, trying to give an idea of something that might be expected to happen in the future. So it's by no means any kind of guarantee; it is, however, an easy starting place before looking deeper into the stock to see if you really could expect a good yield going into the future.
I rent my parents' house in a medium-sized midwest US city; I get around $650/month after property management expenses. After taxes, insurance, maintenance expenses, and the occasional empty period, I'm not getting anywhere near that out of it. But it is still profitable, passive, and the investment needed to increase my income (in other words, buying more property) would not really be excessive.
Pardon me, but that is more or less the opposite of the traditional received wisdom.
High dividend stocks come in two varieties:
* Low-growth, mature, typically capital-intensive companies, especially those in highly-regulated industries. Think power generation, some real estate investments, and so on. Since stock price appreciation is not in the cards, dividends are the only returns an investor can expect and thus are higher. These are typically the safest stocks, although "safety" is relative; stocks are more risky than most other investments.
* A dividend-paying company whose stock price has been depressed, typically due to specific events: poor management decisions, a bad environment, competition, or what have you. These are typically considered "value" investments; the stock price has been pushed down, usually for good reasons, but also usually, too far. The dividend on these is unlikely to be maintainable---management will usually reduce it as part of doing something to get the company back on its feet or whatever. However, the investment itself is not particularly unsafe, in my personal opinion. See Buffet's "The Superinvestors of Graham-Doddsville" for some other people who feel likewise.
The second group is probably not what the poster wants and I would be uncomfortable focusing solely on the first since those kinds of industries are typically economically linked.
On the other hand, you do have to know what you mean by "high dividend". Thanks to the "dividends are doubly-taxed" lunacy, and double thanks to the "growth is everything" mentality, dividends are very low or non-existent. Anything above, say, 4% (to pull a number out of my flying monkeys) is probably a return of capital, not a return on capital; the money you get as dividends includes a partial payment of your original investment and you cannot expect to get your original investment back when you sell it. In fact, it will disappear entirely over time. And anything over 1.5% to 2% (again with the flying-monkey-numbers) probably represents the value investment situation rather than something sustainable. The 6% mentioned by the grandparent probably is indeed pretty risky.
Also, the bond market is usually considered safer than stocks, with a higher interest "dividend". But I don't really know much about that.
This should not be considered financial advice, I'm some random dude on the internet. Look carefully before you leap, try to learn what you're doing before you try to do it, and try to stick with reputable instruction. There is a crap-ton of bad information out there.
[Did I write something particularly controversial?]
Royal Dutch Shell gives a yield of 4.3%. http://ycharts.com/companies/RDS.B/dividend_yield
In 2010, it's yield was 6.5%.
It isn't inconceivable in 18 months time it'll be back near 6%.
It's all about timing.
I'm sure there are other high dividend stocks which are similarly non-risky, high yielding and produces a non-volatile source of income. 6% is on the high side, but isn't impossible.
You also don't have to look for USD denominated investments. The Commonwealth Bank of Australia (Australia's largest bank), gives a dividend yield of close to 5%.
The share price had risen over 50% over the past 4 years so if you had bought it 4 years ago instead the yield would be much more attractive.
Yields are low currently only because the entire market is currently on the high side of the cycle. When it turns back down, that's when you go in and pick up your 6%, which may be 18 months from now (though could be more, or less).
EDIT: If you're really going to do this I recommend diversifying your investment into 3-4 high dividend stocks in different industries - since dividends do vary as the economic cycle in each industry moves up and down.