The choke is a great sound often played in banjo songs. It involves bending a note.  The amount of the bend is usually equal to one semi-tone (or one fret).  This takes practice and can be a little hard on your fingers at first.  In the following example I am bending the 10th fret of the 2nd string towards the direction of my left ear.  Bend that note until it sounds the same as the 11th fret of the same string.  When you have done that then you are playing a full choke. 

The choke technique is fairly difficult and a little painful for your fingers if you are a beginning banjo player.  I would suggest you might postpone this pattern until the calluses on the ends of your fingers are thicker.

left-hand

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right-hand

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Video  Notes: The choke is probably the most difficult banjo technique and yet one of the most rewarding once you have mastered it.  It's hard on the fingers and a great way to speed up the callous developing process.  I generally use my left middle finger for chokes.  Remember though, if you don't bend the string far enough the note will just sound flat and sloppy.

left-hand  

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Video  Notes: The addition of these extra notes is helpful for developing smoothness in your chokes.  I would suggest you practice this pattern more often than just the choke in isolation.