This is a French expression that roughly means “Everything In Place and Ready To Go.”
When I start to make a bread, or any recipe, for that matter, I take out all the ingredients, measure them out and place them in small bowls or on the counter. This way, I know that I have all the ingredients accounted for, they are all in the proper amounts and they are ready to use. As I use them in the dough, I take them off the board, so there is no danger of me missing something, like salt. At the end of the assembling phase, I have no ingredients sitting on the counter. It’s really just a matter of common sense.
For those who need a visual, here’s a picture of the mise en place for the Ukrainian Babka recipe.