In legal terms, to "attest" is to sign a legal document. This signature indicates that the signing party was present for and bore witness to an event, most often the signing of a will or power of attorney.
Attestation is required when witnesses must be present at the signing of a legal document. The witnesses then attest that they witnessed the legal signing of the document by placing their own signatures on it. The signing of wills very commonly requires witnesses to attest, as every state requires at least two parties to witness the proceedings. The signing of a power of attorney and certain types of contracts also commonly require witnesses to attest to them. In this sense of the term, the witness who attests serves no purpose other than to verify that they witnessed all other parties signing their names to the document.
A certifying official may also attest to the legal correctness and authenticity of a document by signing his or her name to it. This is usually done when copies of documents that required witness signings must be made. In this case, an expert attests that he or she has compared the document copy or copies to the original and found them to be completely accurate and faithful. The term is not used in this sense in all states.