Entwine is a keyword ability found on modal spells—spells that normally make you choose one of several listed effects. When you cast such a spell, you may pay the entwine cost in addition to its normal cost. If you do, instead of choosing just one mode, you choose all of them; the reminder text says you choose both or choose all. You make that choice as you cast the spell, so the extra mana is part of the total cost.
In play, entwine turns a flexible one-effect card into a potential two-for-one. The base spell is rarely dead because you can pick the mode that fits, but when you have extra mana, paying entwine can combine both halves for a swing that catches opponents off guard. It rewards holding mana open and planning around your curve.
