Admissible evidence is evidence that a judge will allow into trial, meaning the jury may consider the evidence. Inadmissible evidence is evidence that the judge will prohibit from the trial.
For example, if a defendant is on trial for robbing a bank, a judge might rule that testimony from a witness who saw the robbery is admissible, so the jury will hear this testimony. On the other hand, testimony that the defendant was rude to teachers while he was in high school is probably inadmissible because it is irrelevant to the case.
