Tea is slang for gossip, news, or any other interesting/fascinating story. This slang is mostly used among females to refer to gossips they have. Not every news is worthy of being considered tea, it has to be something mind blowing. For instance, Drake having an out-of-wedlock baby was considered tea before it became general knowledge among his fans.
Spilling the tea literally means you’re going to share some gossip.
Sipping on some tea means you’re listening to some gossip.
There is a meme of Kermit The Frog sipping tea; this meme is often used by individual after reading gossips online.