I never saw that particular expression used but, from what little description we have, this is what springs to my IT mind:

Guard expression is a pre-condition that must be met (expression evaluates to TRUE) before a certain option or course of action becomes available. For instance, you can have a guard expression which will only eneable log-in option 9am-5pm weekdays. Or allows to enter amount to withdraw only if account is active etc.

In a way it's a filter in reverse. Filter allows you to select data to display while guard condition allows you to manipulate that data or, more generally, perform whatever action that condition guards.

Hope that helps but I agree, there isn't enough background for be sure.