Hello,
PSU is power supply unit. Where your power cable is plugged in.
It might be that when you are running more demanding stuff, your power usage is going up,and your PSU can't handle it.So it turns off.Or its warming up too fast. It could be also failing because of its too old.
First thing to do is to try to clean up the inside of your PC. You open your PC case and you with vacuum cleaner try to remove as much dust as yo can.
!!! Attention!!! Do NOT touch anything with it or with your finger. You can burn components inside.
Once cleaned up, close your case and start using your PC as usual. If thishelped, than it was heating too fast. If it didnt't you probably need to change your PSU. Ideally,somebody will borrow you one,so that you can test if it is the real reason.be sure to have coressponding PSU (look for its power and connectors).You can have more powerfull but not less.
If you are not familiar with what I am talking about,please find somebody that knows better in PCs and that person will help you or at least guide your through.
Note: I am note responsable for any damage that might come by doing of actions I suggested to you.