That means "that's just what I / we are expected to do" (even if you think it is special or extraordinary. ) Sometimes people use it to be humble when they do something really heroic. A person might praise a firefighter for saving a child's life, and he might say that "it's all in a day's work" ..that's exactly what he's trained to do.

It means that it's part of your work routine and it's to be expected.