lay a foundation

Definition from Nolo’s Plain-English Law Dictionary

The act of demonstrating to a judge that a piece of evidence a litigant would like to introduce meets evidence rules requirements concerning authenticity and trustworthiness. For example, a medical report cannot be introduced unless the physician who wrote it testifies that he wrote it; and a photograph must be authenticated by the photographer or by testimony that it truly reflects a particular place or event. Even after a proper foundation is laid, however, evidence can still be excluded if, for example, it is not relevant to the point for which it is being offered.

Definition provided by Nolo’s Plain-English Law Dictionary.