Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.

Quicksilver Elemental can gain the activated abilities of any creature on the battlefield that you can target with its ability, even if you don't control that creature.

Quicksilver Elemental gains only activated abilities. It doesn't gain keyword abilities (unless those keyword abilities are activated), triggered abilities, or static abilities.

The granted abilities effectively use "this permanent," rather than "[that card's name]," so you treat the abilities as if they were printed on Quicksilver Elemental. For example, you treat an ability that says "Sacrifice a creature: Nantuko Husk gets +2/+2 until end of turn" as "Sacrifice a creature: Quicksilver Elemental gets +2/+2 until end of turn."

Quicksilver Elemental has a rather tricky interaction with Leonin Bola. You can actually unattach your opponent's Bola as part of the cost of activating the Elemental-copied ability.

You can activate the ability more than once, collecting abilities from multiple creatures (or the same creature more than once).

If you make two copies of an ability that can be activated once a turn, you can activate each of them once a turn.

If you use this card to gain Wall of Deceit's ability to turn itself face down, you will not be able to turn this card face up because it will not have the Morph ability.