A more thorough explanation:

Admissible evidence is evidence that can be presented in court for the judge or jury to consider when making a decision in a case. Rules of evidence determine what types of evidence are admissible, and the judge applies these rules to the case.

For evidence to be admissible, it must be relevant and not outweighed by countervailing considerations. This means that the evidence must be helpful in deciding the case and not unfairly prejudicial, confusing, a waste of time, privileged, or based on hearsay.

For example, if a person is on trial for stealing a car, evidence that they have a history of stealing cars may not be admissible because it is character evidence and not directly related to the current case. However, evidence that they were seen driving the stolen car may be admissible because it is relevant to the case.

In federal court, the Federal Rules of Evidence govern whether evidence is admissible. Each state also has its own rules of evidence for state court proceedings, and many states' rules of evidence follow the Federal Rules of Evidence closely.