A more thorough explanation:

Subscribing witness

A subscribing witness is someone who watches people sign a document and then signs it themselves to confirm that they saw the signatures being made.

When a will is signed, it is common to have two subscribing witnesses present. They watch the person making the will sign it and then sign it themselves to confirm that they saw it happen.

Another example is when a contract is signed. A subscribing witness may be present to watch the parties sign the contract and then sign it themselves to confirm that they saw the signatures being made.

These examples illustrate how a subscribing witness is someone who confirms that they saw a signature being made on a document by signing it themselves.