Definition:
A guard condition is a semantic condition or restriction that is set in square brackets in an interaction operand in a combined fragment. When you create a combined fragment, a guard condition is created automatically. You can also manually create a guard condition in an interaction operand that does not have an existing guard condition.
A guard condition appears at the start of the interaction and contains all the information that is required to make the decision about whether to run the traces in the interaction operand. If the guard condition tests true, the traces run. Because a guard condition is optional, the interaction also runs if no guard condition is specified in the interaction operand.
Further Reading
Book : The Unified modeling Language user guide by Grady Booch, James Rumbaugh and Ivar Jacobson