Webopedia definition:

In programming, a constant is a value that never changes. The other type of values that programs use is variables, symbols that can represent different values throughout the course of a program.

A constant can be

# a number, like 25 or 3.6

# a character, like a or $

# a character string, like "this is a string"

No, you can't use expression or stored procedure call in report parameters page. You can only use this functionality in report designer where you are defining parameters.