This is an essential troubleshooting tool for variables (though there may be others) that will help you nail down the problem.
There are others - simply setting a pre-execute breakpoint on the task immediately following the one which sets the value of a variable will allow you to see the values of all variables' run-time values at that particular point.
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.