JIT means just in time.
If raw materials are delivered weeks in advance, the company must build a warehouse to store the material, hire guards to guard it, and pay maintenance on the warehouse. If raw materials are delivered on the day the company is going to use them, there will be no warehousing expense. This is called Just in Time inventory. The inventory arrives just in time.
Over time JIT has come to mean cost efficient methods including but limited to just in time arrival of inventory.