It's the practice of having all the ingredients for a dish prepared and at hand before starting to cook. Mise en plais is used by chefs, in their fancy women’s and men’s chef aprons, because it helps them to organize their thoughts, assemble everything they need on their station, and reduce stress. They are able to concentrate more on cooking instead of running around trying to find what they need. To this end, they also use waist aprons with pockets where they can easily keep tools with them. The origin of mise-en-plais can be traced back as far as ancient Greece where cooks would keep food preparation tools close by so that they could quickly grab what was needed without wasting time.
Mise-en-place is not just for chefs. Mise en place can be used by anyone who wants to be more efficient and organized in their life. By having all of your tasks prepared, you will reduce stress and make it easier to accomplish what needs done on a daily basis. The only difference between mise-en-plais and everyday people is that professional cooks purchase or create ingredient kits (or prepped ingredients) so they don't have to measure out every little thing before cooking; however, we can still follow this practice fairly easily with premade sauces like marinara sauce from the grocery store. Instead of measuring out each individual seasoning needed for the recipe one at a time, you simply open up the jar and use the marinara sauce as your base.
Mise-en-place has been used by chefs since the 1800s, but it wasn't until after World War Two that it became widely known and accepted among most professional kitchens. The traditional way of life was to always help out with what needed done around the house; however, restaurants were not like households at all, so cooks had to prepare themselves entirely before they could start their days' work beginning from purchasing ingredients (or creating ingredient kits) down to putting on aprons and chopping boards; this is where we get our term 'kitchen prep.' Kitchen prep means to actively prepare for the food preparation process.