The French term in English means “put in place.” In the restaurant world, it means getting all your ingredients out and set up before you begin meal service, preparing garnishes, making sauces, chopping vegetables, cutting and often searing meat that is to be finished in the oven, etc. All the tools and utensils needed during meal service are organized and put in place and the ovens are preheated.
Having everything in place allows the chef to prepare all the dishes he or she is responsible for without having to stop here and there to prepare various components. Mise en place is what allows chefs to crank out plate after plate in the rush of meal service.
This is not technically true, but in home kitchens, mise en place should mean reading all the way through the recipe you are planning to use and getting out and measuring all the ingredients you will need. We get more questions from people who are halfway through a recipe before realizing they do not have an essential ingredient or an immersion blender, at which point we cannot help them and they are – to use another restaurant term – in the weeds.