Altium: PCB rule check violation errors
If you open up your PCB and Violations panel, you can see all your errors. Double click on one of the errors, and a pop up will appear. Jump to the error, so it fills the screen, and the highlight it, so you know exactly where it is.

You can find all the rules within the Design->Rules
Go through the list, and you can find all the rules including the ones in your Message panel.