A shekel is a biblical measure of weight equal to .4 ounces or 11.4 grams.

Shekels were the standard unit of weight like the pound or gram is today.

Shekels are typically used in reference with monetary value - silver, gold, barley and flour.  There is an exception to this when speaking about Goliath.  His armor is mentioned to weight 5,000 shekels and his iron spearhead weighed 600 shekels. 

For those who are curious but don't want to do the math, that's 125 (57 kg) pounds of armor and a 15 pound (6.8 kg) spearhead.