The charges of an item restrict the number of uses of those cast spell properties that specify a number of charges per use (rather than specifying a number of uses per day or specifying "single-use"). Each use of such a property reduces the item's charge total by the specified amount; if there are not enough charges left for a property, that property cannot be used. All properties on an item draw from the same pool of charges, and there is no way to increase this pool unless a module provides a special means to do so (such as making a magic electrifier available).
Items that are not flagged as plot are susceptible to being destroyed when the number of charges runs low. In the simplest case, destruction happens when using a property causes the number of charges to hit zero. Destruction can also occur near zero when there are not enough charges to use any of the item's charge-consuming properties. (That is, destruction occurs when the item appears to be useless and cluttering a player's inventory.) Exceptions to this general rule occur when the item has properties that consume 0 charges per use or that have a number of uses per day, as those properties will prevent destruction if they both have uses remaining and are listed (in the item's description) after the property that caused the charges to drop to self-destruction levels. Thus a property costing 0 charges per use or usable unlimited times per day can always prevent an item from being destroyed due to charge depletion (since it always has uses remaining), provided that property is the last one, but otherwise preventing destruction can seem random.
When charges are increased (by a script), a player may need to rest to take advantage of the addition, as properties with no uses remaining will not have their number of uses immediately recalculated.