Most probably that's about a wasp or a bee sitting right on someone. Не двигайся here means something like get frozen or don't mov a single muscle. А то укусит - otherwise it will bite (or sting - I am not sure which is a proper verb) you. You can say а не то укусит, meaning will be the same.

It is unclear who or what is going to bite the person if he moves. 'А то' means 'иначе' here. It is a fixed phrase that is used as a conjunction.

It means "stand still, or it will bite you" "а то" here means "or", "otherwise" It can be also said Не двигайся, иначе она укусит тебя. We do not know who do not know who said that phrase, to whom it is said and who is that who can bite.

You can also meet the "а не то" version of this construction: Никуда не двигайся, а не то укусит. In fact, "Никуда не двигайся, а не то укусит" = "Никуда не двигайся, а то укусит". А то; А не то = otherwise

Very helpful, thank you:)