To a lawyer, a "disclaimer" is usually a disclaimer of some kind of a liability. A liability, in this context, is an actual or potential legal obligation to pay another person some money as a result of some infringement of that person's legal rights. For instance, a disclaimer might disclaim liability in relation to a particular kind of loss ("Party A shall not be liable to Party B in respect of any reputational loss or damage") or it might cap liability at a certain level ("The maximum aggregate liability of Party A to Party B shall be GBP 1").
Disclaimers can be contractual or non-contractual.
The validity and enforceability of disclaimers is strictly controlled by the law.