Hi smelval,
In the context of a school trip, the word "field" refers to an area where students can study a subject in its natural location, or a location where students can directly observe the studied material.
For example, if you were studying zoology or animal biology, a teacher could take a class on a field trip to the zoo.
The term is also used in reporting the news. A reporter who presents from the scene of a story is called a "field reporter", and the news report that they give is known as a "field report".
Finally, as a third example, if you become a scientist you might engage in "field work". "Field work" is research that takes place directly with the object that is being studied.