A hard-working person puts a lot of effort into their job or studies. They keep going. Teachers, parents, and job adverts use this word as praise. It describes the person, not the job: the work may be easy or difficult; the point is they do not sit still. In British English it is often written with a hyphen, especially before a noun: a hard-working student. You can also say someone works hard — that is the verb. Hard-working is the adjective. Though actually, in very relaxed writing some people drop the hyphen (hard working); in careful school writing, keep it when the words sit before a noun. It is not the same as hard work, which means the task itself is heavy.