"Dryline" is the meteorological term applied to a surface boundary that separates moist air to the East from dry air to the West. Under certain conditions, the dryline serves as a focal point for thunderstorm development in the Plains during the spring and summer months.
A dryline is represented on a weather map by a yellowish-brown line with scallops on the right side of the line, as shown in the image below:
As you can see by examining the above surface map, the dryline separates very dry air (dew point temperatures in the teens, 20s and 30s) to the West of the boundary from very moist air (dew point temperatures in the 50s, 60s and 70s) to the East of the boundary.
Remember, when examining a surface weather map in the format of the one above, the dew point temperature is indicated in green and is located on the bottom left corner of the station model plot, right underneath the temperature reading (which is in red).
As illustrated by the image below (courtesy of UCAR), when the dryline has forward motion, the hot, dry air to the West lifts up the moist air to the East which can lead to thunderstorm development. However, the presence of a capping inversion in the vicinity of the dryline can limit (or even completely prevent) the development of thunderstorms without some other kind of forcing mechanism being present.
When towering cumulus clouds form along the dryline during the late afternoon hours, you can often pick out the boundary on visible satellite imagery as well, as illustrated by the image below: