It assigns to the variable x the value 5. However, there are some important concepts in involved in that instruction which are involved in instructions in general. The first involves the concept of an expression. An expression is any series of symbols which have a value. In the above instruction, the variable x is assigned the value of the expression 5.
Constant values are the simplest forms in which expressions can be put.
The last concept is the concept of an operator. There are however many more operators in LPC, and expressions can get quite complex. In another manner of speaking, operators can be used to form complex expressions. As stated before, all expressions have a value. Now, not all operators do what 1 does.