In trials, each party calls witnesses.
Each party may also question the other’s witness(es).
When you ask questions of the other party’s witness(es), it is called a "cross-examination".
A litigant (or their lawyer) is allowed considerably more latitude in cross-examination then when you question your own witnesses (called an "examination-in-chief").
For example, you are not allowed to ask leading questions to your own witness whereas you can in cross-examination.