Rain gardens are installed in low lying areas of your property in a manner that collects runoff water from roofs and driveways. They are generally planted with native plants which help to hold the runoff water in the soil (and filter pollutants), thus allowing for the water to soak more slowly into the ground. Slowing the flow of water helps to prevent erosion in your yard and from carrying your soil into the local waterways.

The Alliance for the Chesapeake Bay has one of the most informative websites to get you started on rain gardens.   The Alliance's website is: 
http://stormwater.allianceforthebay.org/take-action/structural-bmps/rain-gardens/

This next site, from our neighbors in Montgomery County, MD, provides tons of information, including a tremendous video that shows you exactly how to prepare a rain garden on your property.
www.rainscapes.org

There are other very helpful sites: Click on any one below.
https://www.raingardens.org/
http://livegreenhoward.com/water/rain-gardens-rain-barrels/  
http://www.slideshare.net/Sotirakou964/corsica-river-watershed-maryland-rain-gardens-homeowners-guide

There are several places in Howard County where you can stop by to observe functional rain gardens:

Howard County Conservancy 
Robinson Nature Center