Gridlock is what happens when there's so much traffic on a road or highway that it slows to a stop. If your bus gets stalled in gridlock on the way to school, you'll be late for first period.
While gridlock is commonly used as a synonym for a traffic jam, strictly speaking the term is more specific. During gridlock, traffic is backed up in different directions, so that lines of cars block several intersecting streets. In the worst gridlock, vehicles in the grid of streets are unable to move forward or to back up. Drivers can avoid this kind of gridlock by waiting to enter the intersection until there's room to move all the way through it.