Something laborious takes a lot of time and a lot of effort. A laborious job is the long grind: copying a hundred pages by hand, picking stones from a field, filling in forms one line at a time. It describes the work, not the person doing it. Though actually, it can also describe a way of doing something that feels too slow and too careful — a laborious explanation that takes forever to reach a simple point. It is a little formal; in ordinary talk people more often say “a lot of work” or “really time-consuming”. It is not the same as labour-intensive, which is about needing many workers, and it is not a spelling of laborer, which is a person.