So what you're saying is if I have a total of 5 signatures, I can use just 1 key? What does the M stand for? Thanks for the link, but I'm a bit worried about learning it if it's not used nowadays, kind of just learning about history of something to me.
m-of-n is just a common way of describing multi-sig set ups. The m stands for how many signatures you need, and the n stands for the total number of cosigners. For example, in a 2-of-3 multi-sig, there will be 3 cosigners, with the signatures of any 2 of them needed to spend the coins.
Although P2MS is very rarely used now, the script used in P2MS is the same script which is used in new P2SH multi-sig address, so still worth a read.