The small pocket on the front of jeans began as a way to keep a pocket watch in 1902. It was originally referred to as the "watch pocket," and is the reason many pairs of jeans get the distinctive "five-pocket" label.
Because pocket watches are no longer in style, the small pocket is often referred to as a coin pocket that can hold loose change. This pocket is popular with denim connoisseurs, as it gives jeans a worn, vintage look over time.
When jeans first became popular, they only came with four pockets, with only one on the back. When manufacturers started to add this second back pocket, the jeans began to be referred to as five-pocket.