In a lead-acid battery, discharging turns the active material of the plates into lead sulphate, and charging converts it back. If the battery is left discharged or poorly charged, that sulphate crystallises into a hard, poorly conductive form which no longer converts back under normal charging.
The effect shows up as lost capacity, voltage that sags under load and ever longer recharge times. It accounts for most batteries retired before reaching their theoretical life, especially in stationary installations where the bank spends months on float without cycling.
Incipient sulfation can be reversed with controlled charge and deep discharge cycles, which is the basis of regeneration processes; consolidated sulfation, with plates already buckled or shedding material, cannot. A capacity test is what tells the two cases apart.