Glen Elendra's Answer counters only spells, activated abilities, and triggered abilities your opponents control that are already on the stack. It doesn't prevent spells from being cast or abilities from being activated or triggering later in the turn, and it has no effect on static abilities.

Glen Elendra's Answer can't target mana abilities. An activated mana ability is one that could add mana as it resolves, doesn't have a target, and isn't a loyalty ability. A triggered mana ability is one that could add mana as it resolves and triggers based on an activated mana ability.

Activated abilities are written in the form "[Cost]: [Effect]." Some keyword abilities, such as equip and crew, are activated abilities and will have colons in their reminder texts.

A spell or ability that counters spells can still target Glen Elendra's Answer. When that spell or ability resolves, Glen Elendra's Answer won't be countered, but any additional effects of the countering spell or ability will still happen.

Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities, such as mobilize (from the Tarkir: Dragonstorm release), are triggered abilities and will have "when," "whenever," or "at" in their reminder text.