If someone is laboring, they're working extremely hard, often in a way that makes them physically exhausted. A laboring construction worker needs to take a lot of water breaks on a hot summer day.
This adjective is perfect for describing someone whose job involves physical work, and it's commonly used for the whole group of people who work that way, "the laboring class." It also describes a mother in the process of giving birth: "The laboring cat is looking for a private, comfortable spot to have her kittens." Anyone or anything who's working that hard is laboring, from the laboring engine in an old car to the laboring heartbeat of a nervous dog.