Protection is a keyword ability that stops a card from being affected by anything with a stated quality, usually a color. If a creature has protection from blue, the reminder text says it can't be blocked, targeted, dealt damage, or enchanted by anything blue. So blue creatures can't block it, blue spells can't target it, blue sources can't damage it, and blue Auras can't enchant it. The same works for other qualities, such as protection from black, white, or even wordy (four or more lines of rules text).
In play, protection makes a creature hard to interact with. It can slip past a class of blockers, dodge removal of that color, and survive damage from those sources. Opponents must find answers outside the protected quality or race to win first. It rewards knowing which colors your opponent is playing.
