Avoir du pain sur la planche, literally “to have bread on the (bread)board” means to have a lot to do. Until the early 20th century the expression used to mean that the future was well provided for, that you had enough bread (it would be preserved for a long time before being eaten) stocked up.
The meaning changed afterward. According to one theory, people got used to eating fresh bread rather than dense loaves that kept for weeks. The bread and the board started to evoke the uncooked loaves that the baker lined up on a board to rise. Avoir du pain sur la planche was then referring to the baker who still had a lot of work to do before his bread was ready to be sold.