The fore-and-aft adjustment of the seat determines where you sit in relationship to the crankset (where the pedals are attached), which helps decide how comfortable and efficient you'll be when riding. This is fine tuning of your position and it requires careful eyeballing and some expertise, so you may want to let us do it for you.
If you want to try to check the fore-and-aft setting at home, round up a helper and a plumb line (a length of string with a nut tied on the end will work fine). Place your bike on a stationary trainer making sure that the bike is level. Then put on cycling clothes, hop on and spin in a low gear for a few minutes to loosen up and get comfortable.
Have your helper stand on the bike's right side. Stop pedaling when you you're warm and you feel like you're sitting on your seat's sweet spot, where you spend most of your time. Bring your right crankarm around and have your helper stop the crank when the pedal is at 3 o'clock. He also should make sure that both the crankarm and pedal are level with the ground. Note that for this measurement to be accurate, your shoes must be correctly positioned on the pedals (the balls of your feet should be over the pedal axles).
Holding this position, have your helper place the end of the plumb line on the front of your leg, at a point just below the bony protrusion that's beneath the kneecap. The plumb line's weight should hang over your shoe. Check again to ensure that the crankarm and pedal are level. Now, by looking at the relationship of the plumb line (gauge by the line, not the weight) to the center of the pedal (for reference, use the pedal axle), you can figure what, if any, adjustment is needed.
For most riders and types of cycling, the plumb line should bisect or be slightly behind the pedal axle. If it's ahead, loosen the seat clamp and slide the saddle back (but don't knock it off level). Then check with the plumb line again to make sure the position is correct.
If this procedure sounds tricky, it's because it is. Our experts will be happy to make this adjustment for you if you need help.