A more thorough explanation:

Definition: Lay a foundation is the process of proving to a judge that the evidence being presented is what the person offering it claims it is. Before evidence can be presented to a jury, it must have a proper foundation. The person offering the evidence is responsible for laying the foundation, and if they fail to do so, the judge will exclude the evidence. Laying a foundation can be as simple as an eyewitness stating that they saw something happen and are qualified to testify about it. Other examples include showing that an expert witness is knowledgeable in their field or authenticating physical evidence. However, even if a proper foundation is laid, the evidence may still be excluded if it is irrelevant, hearsay, or could unfairly influence the jury.

Example: During a trial, a witness is called to testify about a car accident they witnessed. Before they can testify, the lawyer must lay a foundation by asking the witness questions to establish their qualifications to testify. The lawyer might ask the witness if they were present at the scene of the accident, if they saw what happened, and if they have any expertise that would make their testimony more reliable. Once the foundation is laid, the witness can testify about what they saw.

Example: In a criminal trial, the prosecution wants to introduce a knife as evidence. Before they can do so, they must lay a foundation by showing that the knife is relevant to the case and that it is the same knife that was used in the crime. They might do this by having a witness testify that they found the knife at the scene of the crime and that it matches the description of the knife used in the crime. Once the foundation is laid, the knife can be introduced as evidence.