In programming languages, an expression is usually part of an instruction which, when run, wiil be calculated or evaluated to give some value (a string, integer, or whatever). This is distinct from parts of the program which actually do something (e.g. assign an expression to a variable, print something, handle flow control).
Depending on the specific language in question, an expression may be made up of literals, operators, function calls, subsripctions, parentheses to determine the order of evaluation, and so forth. The precise details, as well as the order and specifics of evaluation are a central part of the language's definition.