Turtling is the art of waiting. In particular, it refers to the general strategy of waiting for the opponent to make mistakes (as contrasted with continuously baiting them, which falls under the rushdown strategy).
People who turtle very thoroughly aren’t liked by a whole lot of people. Now it’s true that it may be an indispensable tactic depending on the character (to the point of being overly important), but taking these low-risk tactics is often viewed as the mark of a chicken.
Characters that are conducive to turtling tactics include Guile in Street Fighter II and Chun-Li in Street Fighter III.