Katteyomi (勝手読み) is a Japanese go term referring to subconsciously ignoring good sequences for your opponent during reading, making assumptions that lead to things working out better for yourself. Sometimes known as "wishful reading", it has also been translated as "self-indulgent reading."
As with most Japanese terms, katteyomi is not a mere linguistic curiosity, but describes a very real phenomenon that it is important for go players to be alert to: the psychological tendency to favor yourself in reading out sequences. It's possible that stronger players have developed explicit anti-katteyomi measures, where they consider a positive result a reason to explore alternatives that might be less favorable to themselves.
Katteyomi does not refer to the questionable practice of consciously reading and choosing sequences that you think your opponent is too weak to respond properly to.
Katte literally means willful/selfish, or doing something for one's own convenience. Yomi is reading. It means to only be reading your own moves instead of considering the opponent's and therefore all possibilities.
White has failed to anticipate Black's improvement at here. This forces ko when Black answers by capturing at .