Solid trade carts show how much trade income the settlement is currently making.

Greyed-out trade carts show how much the trade income will be increased by the building currently being produced; this is a very good way of assessing which income buildings are most important to build.

Flashing greyed-out carts show how much of the current income will be lost next turn, usually because e.g. you've cancelled a trade agreement.

Does building mines on a resource affect how much merchants make from the resource? I hadn't realised this.