What is the meaning of nap15?

A NAP is a no attack policy (or non aggression pact, or similar). A group of alliances that agree not to attack each other.

NAP 15 would usually mean an agreement between the top 15 alliances in the zone / server. This would be based on the alliance power rankings. Some servers will have NAP10. Some may not have a NAP at all. Agreements are just voluntary and may have different rules. You can ask in Zone chat or ask your alliance leaders who the NAP covers.