I keep seeing these X to do such and such on cards what does that mean? Does that mean pay 10 or does it serve some other function?

"X" is used as a variable in a number of places. For example, on Deep One Assault, the cost is X to "Choose and destroy a character or support card with cost X or lower." So pay one to destroy a cost one character, two to destroy a cost two character, etc.
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Ah ok that makes perfect sense now.