It's a casual, sometimes humorous way to refer to a pig, often used when talking about:
* A pig raised for meat: "The farmer is raising a bunch of porkers for market."
* A large, plump pig: "That pig is a real porker!"
* A person who is considered greedy or gluttonous: This usage is less common but sometimes used humorously.
The word "porker" is generally considered informal and might not be appropriate in all contexts.