Serendipity is when two things that couldn't possibly happen together without it being a coincidence, but then they do, and it isn't. It's serendipity.
Example: let's say all this stuff happens, and you're like "No way! I wasn't even expecting it, all this good stuff that just happened. It's a coincidence!" And then some guy coming by said "No it isn't - it's serendipity." And him coming by and saying that was also serendipity of a sorts, because how would you have known otherwise?
Serendipity. It's hard to explain, except if you see it...and know what it is.