A field is the area around an object where it can exert a force on another object, usually without there being any contact between the objects. We can represent a field as lines showing the direction of the force experienced by another object in the field.
For example a gravitational field exists around a planet and causes there to be a force of weight on any mass placed in the field.
The field can be thought of as the 'cause' behind the 'effect' of the force exerted.
Electric and magnetic fields are used in particle accelerators which have provided the evidence for many of the fundamental particles of the standard model.