Someone who is active moves around a lot or does a lot of things.
If you have an active mind or imagination, you are always thinking of new things.
If someone is active in an organization, cause, or campaign, they do things for it rather than just giving it their support.
Active is used to emphasize that someone is taking action in order to achieve something, rather than just hoping for it or achieving it in an indirect way.
If you say that a person or animal is active in a particular place or at a particular time, you mean that they are performing their usual activities or performing a particular activity.
An active volcano has erupted recently or is expected to erupt quite soon.
An active substance has a chemical or biological effect on things.
In grammar, the active or the active voice means the forms of a verb which are used when the subject refers to a person or thing that does something. For example, in 'I saw her yesterday', the verb is in the active.