An ‘instantly necessary’ box is a clearly-labelled box where you place a range of items, you’ll want to be able to quickly find and use in your new home. This can range from a kettle, tea, coffee etc. and mugs to drink out of; to some toys for your kids to play with while everything else is going on. The same applies to pets!
Another essential might be medicines – ones that you want to be able to instantly locate when needed. Depending on the season and likely weather, it might also include de-icer for your car.
Whatever you quickly require should go into this box; and, of course, it’s the one you don’t pack in the movers’ van but take with you in the car.