If you are 1.5 metres tall and can't swim, and the water is 2 metres deep, you are in trouble because 'you are in over your head'. Another version is to say 'you are out of your depth'.
It's a general way of saying you are in difficulties.
In addition, the implication is that you are involved in something (be it a difficulty or coaching a volleyball team) beyond your expertise, or involving more time or energy than you can actually afford to contribute.