It's building the name of the level for you so you can use the same event for all the levels.
"level" & currentlevel"
it puts the word "level" together with the value of 'currentlevel' which may be 2 for example.
So the word it built was "level2"
when you enter the command 'Go to layout" it happens there is a layout with that name, so that is the layout the project now executes.