First, keep in mind that the random() expression picks a random number - if you input something like random(1) it will pick a number between 0 and 1 (like 0.222, 0.784, etc).
If you read the expression like it was a math problem, you'll understand better.
What happens first is that a random number is generated between 0 and 1 - then this number is multiplied by 4.
Let's say the random number is 0.7!
0.7 * 4 = 2.8, right?
Then we have floor(2.8+1). So of course, 2.8 + 1 becomes 3.8, right?
Then there's the floor() expression that is used to round down a number. If you use floor(3.8) it will round it down to 3.
So, what the code is saying (in this example) is that the game should go to Layout "Lvl 3" - the & is used to append text (in this case, the "Lvl " text to the "3" number", becoming "Lvl 3".
There are multiple ways you could do this. Check out the expression page for more expressions like that: https://www.scirra.com/manual/126/system-expressions