It's a common expression used to:
* Confirm understanding: "Você vai ao cinema hoje? Ta ligado?" (Are you going to the cinema today? You know?)
* Imply shared knowledge: "Ta ligado que a prova é amanhã?" (You know the test is tomorrow, right?)
* Add emphasis to a statement: "O jogo foi sensacional, ta ligado?" (The game was amazing, you know?)
It's important to note that "ta ligado" can be used in informal settings and can sometimes be considered slang.