Elaborating on gtrubetskoy comment, ziplists are doubly linked lists (of integers and strings) encoded into an array of bytes that stores the size of both the next and previous element so you can quickly find the location of the next/previous element. This makes ziplists space efficient and have great cache efficient, but expensive to insert into since you need to reallocate the array and shift a potentially large number of elements over every time you inset/delete. These properties make ziplists useful when they are small, but not so as they get larger, so Redis will switch to different implementations once the ziplists reach a certain size.

Interestingly, Redis lists are implemented as a double linked list of ziplists, where the size of each ziplists is bounded. This gives you good caching as the quicklist is being traversed, while limiting the cost of an insertion. Redis will also compress ziplists not near the edge in order to save space.