The TL:DR on the Jones act is that it requires any goods shipped by water between two US ports to do so on ships built in the US and owned and crewed by US citizens (corps) or leagal migrants (green card holders). It was supposed to invigorate US merchant shipping after WWI, but these days it means that large ships can't make multiple US stops, unless they go to a different country in between.
Some examples: If you are coming from China, a ship can't stop in Hawaii and drop some stuff, then go to LA unless it's US built, owned and crewed. Same scenario for AK on the way to Seattle.
On the other coast, a ship can't stop in San Juan on the way to Miami, nor can one make multiple stops up and down either coast to cut the cost of transport.
In practice since there are *very* few US built, owned and crewed deep water cargo vessels, and US shipyards can't or won't build new ones at competitive prices, it means that US bound cargo is held overseas until there is a full boat's worth going to one port. It also economically incentivizes fewer large ports (so ships can come in and drop the full load) and the infrastructure bottle necks both on the maritime side and the land distribution side that that entails.