When data moves across the internet, it follows a specific set of rules called a transport protocol. Depending on whether you need every single "packet" to arrive perfectly or you just need them to arrive fast, the internet chooses between TCP and UDP.
TCP is the "Reliable" choice. Before any data is sent, it performs a Three-Way Handshake. The sender says "Hello," the receiver says "I hear you," and the sender confirms "Great, here is the data."
If a piece of data goes missing or gets corrupted, TCP notices and asks for a re-transmission. This ensures your email or website code arrives exactly as intended, but that constant checking adds a small amount of Latency.
UDP is the "Fast" choice. It skips the handshake and the error-checking entirely. It simply screams data at the receiver as fast as possible. If a packet is lost in transit, it stays lost.
In a live video call, you don't want the stream to "pause" just to recover a single frame from three seconds ago. You’d rather just skip that frame and keep the video live. That is where UDP shines.
Most modern applications actually use a mix of both. For example, a multiplayer game might use TCP for your inventory and login (where accuracy matters) but use UDP for your character's movement and position (where speed is everything).