What are you meant to do when a guy rests his hand on your thigh?

Ok so i'm extremely clueless when it comes to dating/flirting (or anything in that field), and i've noticed that when you're in that awkward little first date phase, guys often tend to rest their hand on your thigh for some reason. But like what is this actually supposed to achieve? is there an appropriate response or do we both jus ignore it and continue chatting or what?
What are you meant to do when a guy rests his hand on your thigh?
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