When something is backbreaking it requires a lot of physically difficult work. You'll be exhausted after a long day of backbreaking work on a farm.
Backbreaking work doesn't literally break your back, but it will probably make it ache, which explains the origin of this adjective. Digging ditches, moving furniture, and hours of harvesting vegetables are all examples of backbreaking work. You can also spell backbreaking as a hyphenated word too: back-breaking.