What does it really mean when the tornado sirens go off?

I'm one of those stupid people that like to go outside and take pictures of storms and funnel clouds, but I've never had the chance to get a picture of a funnel cloud because each time the sirens go off my mom makes me get in the closet. I go outside and ask my neighbor afterward if he saw anything though because he always seems to be outside when the sirens went off and each time he says no. So I'm wondering what is the criteria to activate the sirens in Texas, does it mean they've spotted a funnel cloud or that one is going to form or there's a tornado coming our way? Please give me good answers I want to know if I'd be safe going outside when the sirens are going off.