A screen test is a method of determining the suitability of an actor or actress for performing on film or in a particular role.
A screen test can also be used to determine the chemistry between two potential actors or actresses, to see if they work well together or not.
For example, a lead for a musical theater-type movie could be requested to sing a popular song or learn a dance routine.
International actors such as Bruce Lee were given screen tests to demonstrate that they are sufficiently articulate in the relevant language.
In Lee's case, for the role of Kato in The Green Hornet, he was asked to converse about Chinese culture in English to judge his grasp of the language, then to demonstrate some martial arts moves to show off his physical skills.