Java APIs

Would someone please help me how to understand what an ACK is in practice? I have an assignment to write a client and server with UDP, but to add on an ACK. The textbook for the course is all abstract theory and doesn't provide a single code example. I understand the idea of what an ACK is, but I'm uncertain what it is in code terms. Is it a specific class that I make an object of? An extended datagram packet? Or just the data within a packet that the application parses?

I've been searching for an example that will help me understand what an ACK is in practice, but I can't find any code example that has some comment saying which part of the code actually represents the ACK, so I haven't been able to figure out what an ACK is specifically.

This will be the first program I have ever written that uses networking, so this is all completely new to me.