得 (děi) is a colloquial, spoken way to say 'must / have to' — it expresses necessity forced by circumstances or logic. It's more everyday than the formal 必须 (bìxū) and stronger than 应该 (yīnggāi 'should'). Unlike 要 (yào), which can also mean 'want', 得 only conveys obligation. Negate it with 不用 or 不必, never 不得.