Cross examination is questioning an opponent's witness at a trial or at a hearing. The opposing sides in a trial or a hearing have the opportunity to cross examine each other's witnesses after the witness's direct examination. For example, when the prosecuting attorney calls a witness to the stand and questions her, the defense attorney will then cross examine the witness and try to get her to say something that casts doubt on her own testimony or brings out contradictions and improbabilities in earlier testimony.
So, if the witness has testified that she saw the defendant walking away from the crime scene, the defense attorney will question the witness about the weather conditions, the lighting, her eyesight, or any other thing that might show that the witness was mistaken in her identification of the defendant. The cross-examiner is typically allowed to ask leading questions and is generally limited to asking questions on matters covered on direct examination.