I use multiple Gmail accounts, but keep them in different Chrome profiles.
If you don't want to do that (e.g. because you want all your email tabs in the same window), you can just use the account switcher on the top right, or just change the 0 in the URL to a 1 or 2 or whatever.
It's an idiomatic term. It means to use the thing you produce yourself. So when, in the case of Gmail, it is slow and painful to use for yourself, you will fix it.
