Can any Europeans explain to me what a TV License is and why you need one?

Some Irish girl was talking about it, but I was so confused and googling it made me even MORE confused. I'm even more confused, because she said some people watch TV without ever getting one. I'm so confused, because in the US we get our channels through private companies and we have to pay a monthly fee or they'll cut it off.

Updates
6 mo
She said there's actually an inspector that comes by and if you don't have one you get fined. Someone else said you even need one for smartphone, tablet, computer.
Can any Europeans explain to me what a TV License is and why you need one?
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