To spank a child is to hit them on the bottom with an open hand as a punishment. The details matter, because they are what separate this word from hit: it is the bottom, it is an open palm rather than a fist or an object, and it is done as punishment — usually by a parent, usually to a young child.

The word also works as a noun: a spank is one such smack, and the action as a whole is spanking, which is the form you will meet most often in reading — "spanking is banned in schools here".

The social weather around this word has changed enormously, and a learner needs to know that. A century ago spanking was ordinary and openly discussed. Today many countries have made it illegal, most child-development researchers advise against it, and in a lot of places saying you spank your children invites disapproval. So the word carries an argument inside it now. When you see it in a news story or a parenting article, it is usually there because someone is objecting.

Two other uses you may run into. In casual sports talk, to spank a team means to beat them badly — "we got spanked 5-0". And the word turns up in adult sexual contexts too, which is worth knowing quietly so that you are not caught out; it means you should not use spank as a playful joke between adults unless you are sure of your ground.

One last note on register: it is an everyday word, not a formal one. A law or a report would more likely say corporal punishment or physical punishment.