MonsterSpeed = 80 is known as a global variable
Global Meaning it can be accessed from everywhere
Variable meaning it can be changed
"Also increase the speed monsters are spawning at" is a note explaining what the
System: Add 1 to MonsterSpeed does
Each time a monster is shot then the variable gains 1. So the first monster will have MonsterSpeed 81 then 82, 83, 84. I imagine somewhere in the game they MonsterSpeed variable is used to set how quickly monsters are spawned but I don't see it in the screenshot and I have never worked with this code.
Hopefully this helps ^^