Pithing Needle stops players from activating abilities of the named card. An activated ability is anything written in the form “[Cost]: [Effect].”
Pithing Needle only stops activated abilities. It doesn’t stop spells from being cast, and it doesn’t stop any other kinds of abilities that aren’t activated abilities (such as triggered abilities, static abilities or replacement effects).
Pithing Needle stops all activated abilities that aren’t mana abilities. An activated ability is a mana ability if it A) produces mana, B) doesn’t have any targets, and C) isn’t a loyalty ability. Any activated abilities that don’t meet all of these criteria aren’t mana abilities, and can be turned off by Pithing Needle.
Some common things you might use Pithing Needle for include:
- Stopping planeswalkers from using their abilities (planeswalkers’ loyalty abilities are activated abilities because they’re written in the form “[Cost]: [Effect]”)
- Stopping lands like Kessig Wolf Run from using their non-mana-producing abilities (Their mana-producing abilities will remain unaffected)
- Stopping Boros Reckoner from using its second ability to give itself first strike (Pithing Needle won’t stop Boros Reckoner’s first ability, because it’s a triggered ability and not an activated ability)
- Stopping Equipment like Ring of Kalonia from equipping a creature (The equip keyword is an activated ability. However, casting Pithing Needle after the named equipment is already attached to a creature won’t unattach it)
Hope this helps! If you’ve got any specific questions about how Pithing Needle works in a given situation, feel free to ask.