To cash out is to exchange poker chips for cash, and is the opposite of buy-in. Because poker is generally played for table stakes, cashing out usually must be done for all of a player's chips (you cannot exchange only some of your chips for cash while seated at the table - you must remove all chips from the table and leave the table in order to remove or exchange any chips).
In commercial casinos, cashing out is only done at an official cashier's cage (which is usually a nearly-literal cage, that uses metal bars to protect the cashier from potentialy unruly patrons). In home games, cashing out is usually done at the "cigar box" where the person who took your money at the start of the game can hand back the money.
While players can usually buy chips at the table in a commercial casino (via a chip runner or sometimes from the dealers themselves), players can only sell chips (cash out) at the official cage.