An FSR, or a Forest Service Road, is a gravel or dirt road constructed to facilitate resource extraction, mainly logging. It is built for a specific industrial purpose, and when that purpose falls into abeyance, so often does the road.
Some FSRs are main lines, and are used, and therefore maintained, on a regular basis. Other roads are used to log a certain area and then decommissioned, often by removing a bridge or digging the road up. Other decommissioned roads will have waterbars, or small ditches, dug into them at frequent intervals to control water runoff. These waterbars can make the road impassable to regular cars.
FSRs often get extremely rough with washboard, especially in a wet spring. They also are subject to washouts, rockfall, water on the road, trees falling across them, and a host of other problems.
FSRs may be used by heavy equipment, including loaded logging trucks. They are also used by loggers who drive them every day and know every turn and bump. Consequently, these workers may drive much faster than you will.
Workers on FSRs in active use keep in contact with each other by radio. If you are on such a road, tuck in behind an industrial vehicle and follow them.
Logging trucks in the bush can carry loads with trees much longer than the truck. These are called sweepers, and can be extremely dangerous to oncoming traffic when the truck goes around a corner. Watch for these.
If you are driving an older vehicle, or have a vehicle from the United States, you might not have daylight running lights. In this case, turn on your headlights. (And remember to turn them off when you park at the trailhead!)
When you park, make sure you are not blocking the road for other vehicles, even if you think no one could possibly go any further. Pull over into a pullout or wide spot.
FSRs can get you a long ways into the bush. Beware that if you break down, getting assistance could be extremely difficult, time consuming, and expensive.