Pool capacity represents the sum of magic energy a caster can channel through himself before wearing himself out (The name comes from the analogy of taking a bucket of water from a pool which Hajime Kanzaka used to describe how spellcasting in the Slayers world works).
Unlike bucket capacity, which is more or less fixed at birth, pool capacity can be expanded as it mainly depends on the caster's stamina. This also makes completely expending it quite dangerous, as the caster may be so weakened that he may fall unconscious or sustain a serious injury. (Lina Inverse has been seen coughing up blood when she overexerted herself with the Ragna Blade. When she cast the Giga Slave against Ruby-Eye it caused her hair to temporarily turn white.)