What is a foreign transaction fee on a debit card?

Could you please explain to me what a foreign transaction fee on a debit card is, and how it works? I've heard of it before but I'm not entirely clear on the details. Is it a fee that I need to pay every time I use my debit card for a transaction outside of my home country? And if so, how much does it usually cost? I'm interested in understanding this better so I can make informed decisions when traveling and using my debit card abroad.