Rick rolling is a weird internet humor tradition.
If you are old enough, you will remember listening to Rick Astley singing "Never gonna give you up, never gonna let you down..." on the radio. Trying to be PC, his voice was big and soulful and kind of what you'd expect out of someone big and soulful looking... when the video came out, quite a few people outside of the UK were shocked by the short, red-haired pasty British fellow they finally saw the voice coming out of.
So... the internet meme goes in a similar fashion... set up something else, let it sink in a bit, then roll it to the Astley surprise. The Rickroll.
Astley was a really good singer... and actually hit the top 10 in the UK with his first 8 singles... he got popular again after people started spreading his video around using it as a joke... the Rickroll.