That $50 would be converted into stock at the time of their next fundraising. Uncapped means there is no cap on the valuation at the time of the next round. So if one of the founders raised money at a $100,000 valuation later, the $50 investor would get 50/100,000th of the shares.
If it were capped, say at a $50k valuation - the investor would get 50/50,000th of the shares, regardless of the valuation.
A convertible note does not buy equity - thats what 'convertible' means - and 'note' means 'loan'. So it's a loan that can convert to equity based on certain conditions.
As I said, all of this is overly simplistic. It's just a 30,000 foot overview.