In classical computing, a bit-flip changes the value of a bit (“binary digit”) from 0 to 1 and from 1 to 0. Despite not using bits, quantum computing borrows the term. Instead of swapping 0 and 1, we swap the labels |0⟩ and |1⟩. For a qubit in a superposition of |0⟩ and |1⟩, the probability of measuring |0⟩ becomes the probability of measuring |1⟩ and the probability of measuring |1⟩ becomes the probability of measuring |0⟩. We unimaginatively name the gate that does this the “NOT” gate, although it has a couple of other names, as well.
But you know how your left pinky finger sometimes reaches for the “A” and accidentally clicks the “Caps Lock?” Well, classical computers and quantum computers make errors, too. You have a “Backspace” key for yours and they have error correction algorithms for theirs.