I am not a physicist, but I looked up the answer to this question a few years ago and didn't fully understand but I think it's something like this.

Mass and energy are equivalent. So if it can lose energy then it can lose mass.

Although famously nothing can escape the black hole, energy can be lost via Hawking radiation. I don't really understand this, but apparently quantum particle pairs are always being spontaneously created in empty space. The pair of particles are opposite: one has negative mass/energy and one positive mass/energy. Usually they are randomly created and then instantly destroy each other, resulting in zero net effect. However if this happens precisely on the event horizon of the black hole, then it's possible for the negative mass particle to be ever so slightly inside and the positive mass particle to be ever so slightly outside. So the negative mass particle ultimately falls in, and the positive mass particle escapes. The result is that the mass of the black hole is reduced (by adding particles with negative mass/energy) and some positive energy flies away from the black hole.

This only kinda makes sense to me, because I'd have thought that there'd be a 50/50 chance of the positive or negative energy particle being pulled in, resulting in a net zero effect in the long term. But I think there's some bias to the negative one falling in more often. idk really.

This is VERY MUCH a layman explanation based on something I half-understood a few years ago, so I'm sure people will come along and correct me where needed via Cunningham's Law. But I think that's the jist of it. I suppose if you wanted a fully rigorous scientific explanation you'd have looked it up instead of asking here.