Translated literally from French "put in place" it's a term used in professional kitchens to mean all of the food components and tools needed for service. More broadly it can mean everything you need to complete any task.
Servers can use it to mean setting a table.
Chef: "is table 7 mised out for entrees?" Food runner: "yes, chef. Table 7 has been mised".
As a cook it can mean everything you need to execute a recipe. For example, if you're frying shallots it can mean that you have your floured shallots ready to drop in hot oil, you have a spider ready to pull them, a sheet tray lined with a sport towel or parchment ready to land them, and salt to season them.
As it most commonly refers to having a station ready for service, mise en place for a sauté station could mean 9 pans filled with ingredients needed to execute your pickups, squeeze bottles filled, sizzle trays and sauté pans stacked, spoons and tongs in a bain marie, salt, oven and salamander are on and cranked to the right temperature, department of health labels are written and posted, your lowboy is stocked with adequate backups for the evening, you have side towels at the ready, printer paper, and plates.