At the very beginning of a player's round, the player can declare to become a resting player and a player can also be forced to become resting by an action that would get a second of his stacks into a fight. After the resting player has fought his next battle he ceases to be resting. A resting player's stacks are ignored for game purposes and can not be attacked.
One of the design goals is that all players meet and play at the same time. This makes it necessary that no player has to fight more than one battle at once. The concept of resting takes care of that. A player does not automatically become resting when one stack is locked in combat, because it is a strong handicap, if you can not recruit. Allowing the player to recruit and at the same time protect him from being attacked, is too much an advantage, so a player becomes resting automatically as soon as a second stack would have to fight. To allow a player to declare himself resting is not really needed in that set of rules, but the pdf version holds a second way to play the game and it is a valuable option there.