Zip is when you take two input sequences, and produce an output sequence in which every two elements from input sequences at the same position are combined using some function. An example in Haskell:
Input:
zipWith (+) [1, 2, 3] [4, 5, 6]
Output:
[5, 7, 9]
The above is a more generic definition; sometimes, zip specifically refers to combining elements as tuples. E.g. in Haskell again:
Input:
zip [1, 2, 3] [4, 5, 6]
Output:
[(1, 4), (2, 5), (3, 6)]
And the more generic version is called “zip with”. You may consider “zip” as a special case of “zipWith”:
zip xs ys = zipWith (\x y -> (xs, ys)) xs ys