A Kara Cancel is a mechanic that started off as a glitch in the Street Fighter III that was later allowed as a mechanic and even used as part of character balancing in Street Fighter IV and Street Fighter V.
While in Street Fighter many attacks require an attack to either connect or be blocked to be canceled, some attacks at certain points can be canceled into other attacks.
Characters with effective kara-cancels such as Ken and Sagat benefit by having increased range in their attacks. Ken is notorious for his kara-canceled throws that make his regular throws rival the range of some of the longest ranged command grabs in the game such as Zangief's light piledriver. Sagat, a character with relatively low mobility, can kara-cancel into his special attacks to increase the range of his Tiger Uppercut or to slowly approach his opponent while pestering them with his projectiles. It should be noted that Kara Canceling doesn't protect the character and has their hurtbox move horizontally to quite a noticeable degree.