A really succinct summary of what P2P is was provided by @JSHarris, here . For completeness, I've copied and pasted an edited version below.
P2P is a means that allows usually small investors to invest money at an agreed rate of return to borrowers that they choose. It's a logical extension of the principle of informally lending a friend some money for a time and then getting it back, with the difference that P2P almost always carries some interest with that borrowing, although often far less than that charged by conventional lenders. P2P also allows groups of investors to amalgamate a number of small investments in a single project, with
[...] P2P there is a formal agreement that the investor will get their investment back after a defined period of time, plus an agreed level of interest (usually, there are schemes that operate within Islamic law that get around the issue of interest, as interest is haram in Islam). (accessed 01/12/2017)