Vigilance is a static ability that means the affected creature does not need to tap when attacking. This allows that creature to be able to block on the next player's turn.
Example
A player attacks with her Griffin Sentinel. This would normally cause Griffin Sentinel to be become tapped, but because it has vigilance, this means that it remains untapped and thus can block any thing that could attack her next turn.
Comprehensive Rules
702.20. Vigilance
- 702.20a. Vigilance is a static ability that modifies the rules for the declare attackers step.
- 702.20b. Attacking doesn't cause creatures with vigilance to tap.
- 702.20c. Multiple instances of vigilance on the same creature are redundant.