Translation:You must go home and rest for a bit.
一定 actually means 'definitely', so adding 要means 'definitely need to' which is better expressed as 'must'.
My teacher in Beijing said 休息一下 doesn't always literally mean rest for "a bit" or "a little". There are a lot of expressions that native speakers add 一下 to without literally meaning "a little" - it's a way of softening a suggestion and an idiomatic way of expressing 休息 - so here can just mean "go home and rest". Anyone else been told that?
Me too has been told that. Duolingo has to improve definitely more than 一下！