Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities (such as equip and unearth) are activated abilities and will have colons in their reminder text.

Myrel, Shield of Argive's first ability prevents opponents from activating abilities of permanents on the battlefield that are artifacts, creatures, and/or enchantments. It doesn't prevent activated abilities from cards in any zone other than the battlefield. Notably, this means that your opponents may still activate abilities of cards in their graveyard (such as unearth).