This means that evidence cannot be allowed to be presented to a judge or jury.  The reasoning for this is that if does not meet the state or federal (or military) requirements that allow evidence to be presented in court.

This term is the opposite of admissible evidence.  The main reason evidence may be deemed inadmissible is if it falls under the hearsay rule.  This basically states the quoted testimony from a witness will not be allowed because the witness will not show up during the court trial, and their testimony cannot be proven to be reliable and/or helpful in determining motive.

Other reasons for inadmissibility is when the expert advice of a testimony is deemed to be opinion rather than fact.  Lastly, if there is a defect in the evidence, it can also be deemed as inadmissible evidence.

Example

During a Guam divorce case, a deposition is called during the discovery process.  During the deposition, the opposing Guam lawyer asks a question that was not clearly stated.  The witness replies but appears to not understand the intent of the question.  Nobody catches it, and the deposition questioning continues.  During the trial, the judge deems that the answer given by the witness was inadmissible and therefore should not be shown during the trial.