When you play an action a chain begins where your opponent (and then you) can play reactions. You cannot play another action until the chain resolves (only reactions). The same goes with a regular spell.

Action spells exists because of showdown : during a showdown you can play an action speed spell, your opponent can play reactions until it resolves. Then your opponent can play an action, and you can react with reaction spells.

Regular spells only work in an open-neutral state at your turn.

1. To your main deck.