It's a casual and informal way of expressing that you understand or are knowledgeable about something. It can be used in a variety of situations, such as:
* When someone is talking about a topic you're familiar with
* When you're being asked to do something you're capable of
* When you're confirming that you understand something
Example:
* Friend: "Let's go see that new movie."
* You: "Essa eu manjo! I've heard good things about it."
Note: The word "essa" is a feminine singular demonstrative pronoun, similar to "that" in English. It is used to refer to something that is close to the speaker or listener, and is being discussed.