Moderators can edit users profiles but they only have restricted number of things they can edit compared with when admins edit users profiles and they can't edit admins profiles.
They can only do things like change users preferences and suspend users, they can not change user groups, delete accounts, etc. this then takes less presure off the admin and allows things like if a spammer is on the board and there is no admin around to stop them a modertor can suspend the user to stop them, the admin can then decide whether to reactivate the user or delete thier account.