A 'glitch' is where something goes wrong with something electronic. Usually something to do with the program, rather than the hardware. So something that's 'glitching' isn't working how it should. Your computer date sometimes switching to the wrong year (without you setting it as that) would be a glitch.
'Lag' is where there's a delay. It's used outside of computing, but tends to mostly be used where a poor internet connection is causing the delay. So if your computer is 'lagging', then you'll have a hard time playing Fortnight.
On a related note there's also the terms 'bug' and 'crash'.
A 'bug' happens when there's an error in the code of a program. A bugged program may cause glitches or crashes.
A 'crash' is where something happens that totally stops what you're doing. Maybe it when it 'crashes' that makes the computer shut down or reboot, maybe just the program closes or stops doing things, or maybe your computer 'freezes' and stops responding to anything until you turn the power off.