You actually didn't full combo the map.
You failed to clear some sliders, which doesn't break your combo but doesn't add an extra point to it:
A slider is composed of a starting point, counting as +1 in your combo, sometimes middle points, for long sliders, each counting as +1 in your combo, and the ending point, also adding +1.
A long slider can be represented as such:
(start) (middle) (middle) ... (middle) (end).
In your case, most of the sliders are (start) (end), they are not long enough to imply some middle points.
Failing to hit a (middle) point of the slider results in resetting your combo, it is counted as a failed note.
But the trick here is that failing to hit the (end) point doesn't reset your combo, but it doesn't add the expected +1 and gives a +100 instead of +300.
You can see this at 0.49 and 1.27 in your video, you start your slider, move, then you release your key (or mouse button) before completing the end of the slider. Resulting in a +100 and not increasing your combo: you haven't fully completed the slider.
You did it twice, resulting in having a combo of 522 instead of 524.
In Osu!, pp stands for performance points. There are two main contexts to consider pp in, at the beatmap level and at the account level.
On an individual beatmap, pp is based on the difficulty of the song and one's accuracy score on the song. One can try over and over to improve their performance, but only the best performance on each beatmap will count.
On an account, pp is based on the individual amounts of pp from each song, however, there are diminishing returns to encourage great performances on a few songs over mediocre performances across many songs. One only gets full pp from their very best performance.
To use the currently top player (as of May 23, 2016) as an example, Rafis sits at 12,204pp. Since players are ranked by pp, we know that's more than anyone else. Scrolling down in his profile and opening the "Top Ranks" section, we see that the song granting him the most performance points is Fujijo Seitokai Shikkou-bu - Best FriendS [Insane]. His 98.98% accuracy grants him 715pp. The next song, rated at the song level at 708pp, is granting his account 673pp because the second best song has a weight of 95%. As you go down through the songs, the weights continually decrease, with the total contribution after applying weights eventually summing to the overall account score of 12,204pp.
https://osu.ppy.sh/wiki/Performance_Points
Best Answer
I believe it's centred on precisely when the approach circle touches the hit circle or slider. There is a window on either side of that time where you can hit it and score 300. Without mods, this will range from 19.5 ms to 79.5 ms on either side. This gives a total window of 39 to 159 ms. A full chart is available on the osu! wiki.
It's actually possible to enable a view to show this more immediately in-game. In the options menu, set the Score meter type to Hit error, and a bar will appear at the bottom of your screen that reflects your most recent hits, relative to perfect timing.
Note that the precision required varies by the map settings (specifically, the OD or Overall Difficulty), and that sliders are more forgiving than hit circles in terms of timing precision.