Trade is where two governments, send eachother resources, items, etc. In return they either recieve currency or another material good. Most trade takes place using trade-ships, cargo-ships, or carriers. Although these are vulnerable to attack by Pirates and or a enemy government, its best to escort them with fighters or another aeronautical military entity.
Depending if the player is there only can you pirate their ships, or attack them. Although you can move your forces near and when they log-on and are aware of the event about to take place then you may attack. Trading is usually good to boost trade, growth, and prosperity of a government and its beings.
Trade isn't to be taken lightly as trade-ships could hold very valuable items, but its better to not leave a trace because you'd be throwing yourself into a un-wanted war if you randomly are to attack a trade route.