More specifically, it is the past tense of the verb "to be".
Here are some examples of "was" used in sentences:
* She was happy. (The verb "was" is used to describe the state of being happy.)
* The dog was barking. (The verb "was" describes the dog's action of barking in the past.)
* We were at the park yesterday. (The verb "were" is the plural past tense of "to be" and is used to describe the location of "we" in the past.)