The creature can’t move, unless it teleports or is pulled, pushed, or slid.
An immobilized creature can’t reposition itself on the battle grid, but it’s not paralyzed. It can still stand up when it’s prone, pull an item from a backpack, or attack normally. It might have taken a serious, but temporary, injury to the legs, or it could be stuck to the ground by a huge cobweb.
A creature wrapped up in animated vines might be restrained instead (see below); that condition imposes more restrictions on the creature’s ability to attack and defend itself. A creature that is actually paralyzed, such as by the bite of a ghoul, might be stunned rather than immobilized or restrained.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 221, Rules Compendium, page(s) 232, Monster Vault: Threats to the Nentir Vale, page(s) 123.