Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Both implement List, but their storage models make different operations and traversal patterns expensive.
ArrayList stores references in a resizable array, so indexed reads are fast and iteration has good memory locality. LinkedList stores linked nodes, so finding an element by index requires traversal. Inserting into a linked list is cheap only after code already has an iterator or node position; searching for that position is still linear. For most application lists, ArrayList is the default because reads and iteration dominate.