(Junior high/ high school level)
When a bridge is being built a lot of work goes into figuring out the forces at every point in the bridge. This is important information for the engineers because they need to know how strong the cables have to be. Do you think the Golden Gate bridge could be built using kite string? I doubt it. But you wouldn't want to use cables that were 10 feet thick either -- they would be too heavy and too expensive. An important concept the engineers use to help figure out the forces is the idea of static equilibrium. Static equilibrium simply means that something is not moving. Like the bridge, we hope.
We can get a feel for why something is in static equilibrium by just thinking about a rock on a table. If you put a rock on a table it won't move unless you make it move by applying a force to it (like pushing it with your hand). The reason the rock doesn't move is that all the forces on it are balanced or act in a way that they cancel each other out. The rock has a weight which is a force caused by the gravity of the earth. This force pulls straight down on the rock. So if the table wasn't there, the rock would fall. The rock doesn't fall because the table is also exerting a force on the rock. This force is upward. So the two forces on the rock, the one due to gravity (down) and the one due to the table (up) are equal and cancel each other out and the rock stays put.
By realizing that every point of the bridge is in static equilibrium the engineers can find the forces anywhere on the bridge and pick the right cable (or other materials) for the job.