Type of Action: Move Action
Walk refers to moving along the ground at your listed speed. For most size-medium creatures, this is 30 feet. Moving like this is a cautious walk, and is primarily used during combat. Despite this, walking provokes attacks of opportunity from enemy creatures, if you leave a space they threaten.
Characters can only walk through unoccupied spaces or spaces occupied by allies, and any walk must end in an unoccupied space, or the creature is subject to the Squeezing rules.
The terms "base land speed" and "move" are often used interchangeably with "walk". It may be useful to note that "move" is a type of action, and "walk" is a way to use a "move action" (to move along the ground at your listed speed).