A spell is a series of words that has magical powers. If you’re under a spell, then what you do is out of your control — your thoughts and actions are dictated by the spell.
Spell can refer to the magic words you say, or it can describe being under the influence of those words. You probably don’t encounter many magic spells in real life, but the word is often used figuratively to describe those times when you feel like a magical power controls your action. Of course, this word can also mean the verb "to spell" which refers to reciting the letters in a word, like the competitive spellers in a spelling bee.