I just found out that I had a loose spoke and I used an adjustable wrench to tighten it back up. Just to check, I span the rear wheel to see if everything was fine. I discovered that there is a slight drop where the loose spoke was. What is the problem? Thanks!
If you want to tackle this yourself without buying the truing stand and tensiometer, tighten the spoke(s) at the drop until they are the same tightness as their surrounding spokes. To determine this without buying a tensiometer, pluck that spoke and the surrounding spokes; they should have a similar tone.
How's the side-to-side play, by the way?
If you get the replacement spoke's tone pretty close to that of the surrounding spokes and the wheel still has the dip, tighten the spoke at the dip about a quarter turn, THEN loosen the opposing spoke that is on the other side of the wheel by the same amount. Hopefully it won't come to that, because when you start making corrections like this without having the tools, or some experience in wheelbuilding & repair, you might end up spending more time learning than riding.
There are four parts to building or truing a wheel, radial, lateral, centering, and tension. The lateral alignment is what most people consider as problematic as that is what causes rim brakes to rub. Centering is how close to dead center the rim is in relation to the axle midpoint. Tension is the tightness of the spokes (which alters their load bearing capability and longevity). Radial alignment is the problem you are seeing. It's sort of like pulling a drawstring on a bow, the tips flex inward more the harder you pull. With a wheel, getting an even amount of pull across the spokes will get the whole wheel to roll in a circle.
A tiny hop won't affect performance much (as your tire will soak up the error) but a large one can lead to failure of the tire bead to stay in the rim under pressure, leading to a blowout.