If you don't play the card, it will remain exiled.

An "equipped creature you control" is any creature you control with one or more Equipment attached to it.

If you cast an Equipment spell this way, you may choose to pay its mana cost if you want to.

Nahiri's last ability will trigger once for each equipped creature you control that attacks, no matter how many Equipment are attached to any of them.

Unless an effect allows you to play additional lands that turn, you can play a land card exiled with Nahiri only if you haven't played a land yet that turn.

One reason you might want to pay its mana cost is if there's an {X} in its mana cost. If you cast such a spell without paying its mana cost, you must choose 0 as the value of X. (If you pay its mana cost, you may choose the value of X as normal.)

You may play the card exiled with Nahiri's ability even if Nahiri or the equipped creature leaves the battlefield or you lose control of them.

Playing the exiled card follows the normal rules for playing that card. Unless you're casting an Equipment spell this way, you must pay the spell's costs, and you must follow all applicable timing rules. For example, if the card is a creature card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.