さもないと means 'otherwise' or 'or else'. It is used to present a consequence that will occur if the previous statement is not followed. It is often found in warnings, threats, or urgent advice. The tone is fairly formal and literary, so it appears more in writing or in dramatic speech than in everyday casual conversation.

In structure, さもないと connects two clauses: the first clause states a condition or command, and the second clause states the negative result. For example, in '急ぎなさい、さもないと遅れますよ' (Hurry, or you'll be late), the first part is a command, and the second part warns of the consequence. It can also follow a statement of fact, as in '今すぐ行かなければならない、さもないと終わってしまう' (We must go now, or it will be over).

This expression is not used in casual chat. In spoken Japanese, you would more likely hear 'じゃないと' or 'ないと' instead. For instance, '急がないと遅れるよ' is a natural casual way to say the same thing. But in formal writing or speeches, さもないと is appropriate.