I saw in a sentence: We do our bit, don't we?
Whats the difference between doing your bit and doing your best? Are them interchangeable?
Sounds like typical British self-effacing modesty.
No doubt the speaker does do his best, but he's not going to say so. He is merely going to point out that he attends to his part in the operation, and implies that this part is not particularly big or important, but that it is his lot to do it.
The expression 'do [one's] bit' dates from at least World War I, and expresses the idea that each person's contribution to a larger effort is important. It was used by the UK and US governments in that war in a number of ways, among them publicity campaigns urging people to do things to help the war effort, including saving food. It became popular again in World War 2 and became part of popular speech in the UK and possibly elsewhere.