Shotgun choke is the constriction at the muzzle end of the gun that tightens the pattern of pellets. In an average cartridge there are approximately 300 pellets, so how wide the shot pattern or how restricted is a matter than will make all the difference to your shooting.
Choke is one of those things, like gunfit, that should be visited occasionally and put out of mind once an informed decision has been reached concerning what best suits your needs.
You should take your gun to a pattern plate and shoot it at different ranges – 20yd, 30yd and 40yd – using the cartridge you prefer. You hope to see an even pattern without too many clusters, gaps or excessive central concentration.
If there are holes a bird could fly through – a 5in circle test is sometimes applied – or if the pattern is obviously too tight, your shotgun and its chokes may be working against you.