In the English language, water is often used as a metaphor for a difficult situation. To be literally submerged in water over your head, especially way over your head, is to be in a position that is both out of the norm and potentially highly dangerous. This idiom transfers that literal meaning to figuratively refer to being deeply involved in a difficult situation which you lack the skills, knowledge, or resources to be able to cope with or escape from.
In simple terms, to be/get 'in way over your head' means to find yourself in a situation that is too much for you to handle. This expression can be shortened to 'in over your head,' and, as the video shows, is common in both a social and professional context. Phrases with a similar meaning include 'out of one's depth,' and 'out of one's league.'