A piece of documentation, product data, a sample, a shop drawing, a certification, that a contractor sends to the design team before buying, fabricating, or installing something. Its whole job is proving that what the contractor plans to use actually meets what the specifications require.
Specs describe performance in general terms, often enough that more than one product could technically qualify. The submittal is the point at which a contractor has decided on a particular item, and the design team can opt to approve that selection or opt to provide feedback. If the submittal step is skipped, there is no record to confirm if the product was checked against the spec before it was delivered to the site.