I was reading the Pathfinder Player's Guide, and I found a lot of magical items and clothing that required command words to activate. What are these command words? How do they work?
Rules for Command Words are in a few different places. What it is, is simply providing an avenue for the GM to restrict use of items that the party finds. Usually it doesn't matter, since the party will typically have someone who will identify what each magic item is and does with Detect Magic or similar.
Most importantly, from Magic Items:
Command Word: If the activation is on command or if no activation method is suggested either in the magic item description or by the nature of the item, assume that a command word is needed to activate it. Command word activation means that a character speaks the word and the item activates. No other special knowledge is needed.
Sometimes the command word to activate an item is written right on the item. Occasionally, it might be hidden within a pattern or design engraved, carved, or built into the item, or the item might bear a clue to the command word.
The Knowledge (arcana) and Knowledge (history) skills might be useful in helping to identify command words or deciphering clues regarding them. A successful check against DC 30 is needed to come up with the word itself. If that check is failed, succeeding on a second check (DC 25) might provide some insight into a clue. The spells detect magic, identify, and analyze dweomer all reveal command words if the properties of the item are successfully identified.
One other important note: saying a Command Word is always a Standard Action that activates one item. No matter how many items have the same word, or how easy the word is, it takes up your Standard Action.
Activate a Magic Item (from Gamemastering):
… Certain magic items, however, do need to be activated, especially potions, scrolls, wands, rods, and staves. Unless otherwise noted, activating a magic item is a standard action.
Finally, the actual word(s) required are determined when each magic item is created.