Picture Source: Getting Stronger by Bill Pearl and Gary T. Moran, Ph.D.

A straight bar is like a barbell but shorter.
Starting position- hold the bar in front of you in the underhand position (palms facing away). 
Movement- Curl the weight up to your chest without moving your elbows (keep them at your side).  Then back down.
Note- Keep a slow controlled movement.  Reverse curls are a great variation exercise. Do the same movement with an overhand grip instead.