What is it?
A barbell bicep curl is a strength exercise which works primarily your biceps, but also trains muscles in your forearms and shoulders as well.
How to do it:
You can perform regular barbell curls by grasping a barbell with an underhand grip, and with your hands separated by a shoulder-width distance. Allow the barbell to hang at arms length in front of you, with your hands, arms, and shoulders aligned. While focusing on your biceps and keeping your upper-arms still, lift the bar upwards to your chest. Once you've lifted the bar as high as you can, squeeze your biceps and then begin slowly lowering the bar down until your arms are nearly straight, again.
Don't lean backwards or swing the weight during barbell curls as it causes you to use your back to lift the weight, while swinging the weight causes you to cheat by using momentum to get the bar to the top of the lift.