The particle が is known as the subject particle and serves as a marker for the subject of a sentence. Although the terms topic and subject are frequently used interchangably in English, there is an important distinction in Japanese. Unlike the topic particle は, the subject particle が has many useful functions that differentiate it. It highly recommended that you study the use cases and functions of が below to gain better familarity with it. 

Sometimes it might be difficult to determine whether to use the subject particle は (wa) or subject particle が (ga) when building Japanese sentences. Fortunately, in many cases you should still be able to be understood regardless of your choice. However there are some circumstances where you must use が (ga) otherwise the sentence will be grammatically broken or significantly change the meaning.