Expression is anything that matters. A function call considers to be the expression too.
There is no traditional arithmetic operators in the ScraperLab scrip, but there are logical operators, and, as a consequence, there are logical expressions with values 1 (if the expression is true) and 0 (if expression is false). Not logical expressions could be used only in the construction as if else.
There are only predefined functions in ScraperLab Script which could be used in your own script. To call the function you need to specify its name and parameters in parentheses, separated by commas, which would be passed to the function.
Any expressions must be separated from each other by the semicolon (;)