What does the phrase pigs fly too mean?

The phrase "when pigs fly" is a response equal to "it will never happen", or "I will never do it". The fact is that pigs (large and heavy creatures) are never likely to gain the power of flight, so it is equally unlikely that the activity will ever occur.

Nearly synonymous is the phrase, "when Hell freezes over".

Example (here used as sarcasm):

"Do you think I could win the race?"

"Sure...when pigs fly."