Routers allow player to send items across other conveyors or conduits in three different ways. It can store up to 20 items. This is useful for splitting items from one conveyor into multiple turrets.
Router blocks can be placed at the intersection of conveyor belts or conduits. However, liquid routers are unable to transfer items, and regular routers can't transfer liquids. Routers can't act as bridges like Junctions. Routers also can't be rotated.
Not for merging
Routers are not for merging due to their slow output rate. The picture shown is why not using router for that purpose is recommended. It is advised to use conveyors for merging, although there are better techniques. See Tunnels or Sorters for more merging techniques.
Do 'chain' routers
Router can keep up to 20 items at a time. This could delay the distribution to the smelters or crucible if you chaining the router from the sources. Below is the example of router chaining and the solution to encounter the problem.
This was tested by SNEAKY_WYZRD in Mindustry Discord. As the animation shows below, the 10 routers (at the third line) took a long time for the irons to reach the end. This can acts as storage or overflow gate for distribution networking system. But as transfer system to/from smelter is not a good option.