A bridge is a structure to cross an open space or gap. Bridges are mostly useful for crossing rivers, valleys, or roads by vehicles but people have also used bridges for a long time for walking.
There are six different types of bridges:
There are other types of bridges and some contain elements of a few of the above ones so feel free to get a little creative with your task.
Some things to consider
This is a video the Multi Class made a few years ago showing some of the bridges they built in Minecraft.