Ampicillin ("AMP" symbol in the lab) is a β-lactam class antibiotic that acts by disrupting the synthesis of bacterial cell walls. Ampicillin is derived from penicillin, and is used to select for cells with plasmids containing a gene producing beta-lactamase that breaks down ampicillin. It also helps to prevent unwanted growth of other cells on the plates. If it is warmed too much it is destroyed, so media has to cool to 60 C (ideally 55 C) or below before adding ampicillin.
The final concentration of ampicillin should be 50 µg/ml or 0.05 mg/ml (divide by 1,000 to convert µg to mg). This is 0.005 g per 100 ml solution. A 100X stock can be made up with 0.5 g / 100 ml (of 50% EtOH to promote dissolving the ampicillin powder) and 1 ml stock added to 99 ml solution. (Aliquot the 100X ampicillin solution and keep frozen so you only have to take out and thaw a few milliliters at a time.)