Can you activate a mana ability during the resolution of a spell or ability to pay for something?

Normally, players cannot cast spells or activate non-mana abilities in the middle of another spell or ability resolving. However, mana abilities are a special exception to this rule. Whenever a cost or payment of mana is required during resolution, the affected player may activate mana abilities at that time to generate the needed mana (CR 117.6, CR 605.3b).

A mana ability is defined as an activated ability that (1) could put mana into a player's mana pool, (2) does not have a target, and (3) is not a loyalty ability — OR a triggered ability that triggers from a mana ability and could add mana to a player's mana pool (CR 605.1). Because mana abilities don't use the stack, they resolve immediately and don't allow other players to respond (CR 605.3a).

This matters most when a spell or ability has an additional cost mid-resolution. For example, if Reshape is resolving and instructs you to sacrifice an artifact and pay X mana, you can tap lands or activate your Elvish Mystic's mana ability right then to produce that mana before continuing — even though the spell is mid-resolution.

One important nuance: only mana abilities can be activated this way during resolution. You cannot activate other types of abilities (like equip abilities or planeswalker loyalty abilities) during the resolution of another spell or ability, as those would require priority (CR 116.1, CR 601.2).