Initialism for "speak with you later". It's a way for the speaker/writer to politely indicate that the conversation has come to a natural close or that he needs to focus on an activity other than the conversation but does want to finish the conversation later or will get back to the listener/reader later with the desired information.

It's better than "talk to you later" (TTYL) because SWYL implies actual dialogue or conversation between the two people ("speak with you" vs. "talk to you").

It also can be a more formal or businesslike way to bring an IM, email, or SMS text conversation to a close.

When you eat dirt for the first time and leave your family to build a shack in the woods

term meaning "sex with you later"