If a molecule is free, it has not bonded with or combined with another compound. It is therefore available for sanitizing. When free chlorine molecules encounter and destroy a nitrogen or ammonia containing compound, they combine with them to create a combined chlorine compound, or a chloramine . If you smell a strong aroma of chlorine in and around a pool, chances are it has high combined chlorine levels. This level can also be tested with a DPD test kit which measures total and free levels separately and allows the tester to determine combined levels by subtracting the two.
Total chlorine is simply the sum of combined and free levels.
These are synonymous terms for oxidizing everything in the pool. By raising chlorine levels ten times the level of chloramines, a threshold is reached called breakpoint chlorination . When this is reached, something of a shock , or perhaps more akin to a lightning bolt, rips through the water, slashing and burning everything in its path. Others recommend it once every few weeks, whether it needs it or not. You may use your senses to determine the need for shocking.