Licenses?

Just wondering what the diffrent licenses mean in real life what can you do for example with a private or student etc...
Flight Simulator and other chat
https://forums.simviation.com/phpBB3/
https://forums.simviation.com/phpBB3/viewtopic.php?f=34&t=96480
Ive been wondering, If you want to get a multi engine rateing do you need to be instrument rated first?