What is deposing?

Here are the meaning of the terms you're looking for.

DEPOSE- To testify or give ( evidence, etc) on oath, especially when taken down in writing makes a deposition.

DEPOSING - It is an act of testifying or providing (evidence, etc.) on oath in written form.

DEPOSITION - A deposition is a witness's sworn out-of-court testimony.
It is used to gather information as part of the discovery process and, in limited circumstances, may be used at trial. The witness being deposed is called the "deponent."