Declares labels
Declares the labels val and print. The first which points to a memory address that holds the value 'L' and the second that points to a memory address that holds a piece of code that prints val to the screen.
As you can see in the example, it is possible to dereference a label to get the underlying value, which is done using angle-brackets [val]. This opens the door for things such as pointer arithmetics, which we will explore in a later example.
When declaring a one-liner label, such as val, it is possible to skip the :, like val db 'L'. I would not recommend this, since it makes the code less clear and opens up for errors if you were to add additional lines.
A label in assembly is a symbolic way to write memory addresses (source). However, for someone new to assembly (like me), the syntax of it can be a bit confusing since it looks like either a variable declaration (val) or a function declaration (print), which of it is neither. To re-iterate, a label is simply a symbol (name) that points to a place in the code (memory address).
When a program is executed and hits a label, the execution of the program will continue as normal. It is not "going into" the label, not "skipping it", or anything like that. There are ways to go to a label, such as call or jmp, and we will explore this in a later example.