Kara cancelling is a technique where the initial frames of a normal move are cancelled (before it comes out) into a second move. This changes the position and properties of the second move. The name is derived from the Japanese word 空 (kara) which means 'empty'.
This mechanic was used a lot in the OG fighting games like Street Fighter III: 3rd Strike, but was also later used in Street Fighter IV and V.
In the case of Street Fighter III: 3rd Strike, one of the more commonly used kara cancels is Ken's c.MK move cancelled into a DP+LP (or DP+MP in the corner). This allows Ken to juggle a second uppercut after the first light uppercut connects, which becomes his most damaging non-super combo.
Kara cancels require frame perfect timing, and not only one or two, but THREE of these are demonstrated in a row here using the KeyCade KKL controller, performed by BoAngel: