Copied and pasted per sticky (what to test for and what it means):
FC - Free Chlorine
Free chlorine shows the level of disinfecting chlorine available to keep your pool sanitary. FC should be tested and chlorine added daily, unless you have an automatic feeder or SWG in which case you can test it every couple of days. FC is consumed by sunlight and when it breaks down organic material in your pool. The level of FC you need to maintain depends on your CYA level and how much your pool is used. See The Pool Calculator, Chlorine/CYA Chart by Chemgeek, or Ben's Best Guess Chart for guidelines on the appropriate FC level to maintain based on your CYA level. It is important that you do not allow FC to get too low or you run the risk of getting algae and/or having an unsafe pool.
FC is raised with bleach, trichlor tablets/pucks/sticks, dichlor powder, cal-hypo powder/capsules, or lithium hypochlorite. Only use bleach without any additives, typically labeled unscented or "original scent". Trichlor and dichlor also add CYA and lower PH. Cal-hypo also adds calcium. Lithium hypochlorite tends to be quite expensive. It is most efficient to raise the FC level in the evening since none will be lost to sunlight until the next morning. FC normally goes down by itself. If you are in a hurry you can lower FC with a chlorine neutralizer.
CC - Combined Chlorine
Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. CC causes the "chlorine" smell some people associate with chlorine pools. If CC is above 0.5 you should shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. CC will normally stay at or near zero as long as you maintain an appropriate FC level and the pool gets some direct sunlight.
Potassium monopersulfate (a common non-chlorine shock) will often show up on tests as CC. There is a special reagent you can get to neutralize the potassium monopersulfate so you can get a true CC reading.
TC - Total Chlorine
Total chlorine is the sum of FC plus CC. Inexpensive chlorine tests, such as the common OTO test which shows TC as different shades of yellow, normally show TC because it is easier to test for than FC and CC. In normal operation TC can be used as if it was FC because CC is usually zero. However when you have algae or some other problems, CC levels can be significant and TC becomes useless.