I'm stupid. What is MB4 and MB5?

[KonaManta]

Newish/returning player on PC, I'm not sure if something changed since I last played or if I just forgot. Some of the key bindings are to something called 'MB4' and 'MB5' and have a weird logo that looks like a pill next to a mouse, and other than that is totally unexplained. I can't use certain weapons like the longsword without them.

I tried googling it and didn't get much results - is this some monster hunter specific thing? Can someone give me a hand here? Hahaaaa this is making me feel stupid

[Loxly]

Mouse side buttons

[Willow]

Mouse Button 4 and 5 - thumb buttons on your mouse (if you have them)