Last year, many in the Texans fanbase was surprised by the defensive prowess of undrafted free agent (UFDA) linebacker Dylan Cole from little known Missouri State.
Undrafted Free Agents are rookies who weren’t selected in the NFL Draft, are then signed by a team as a free agent, and make the roster usually by usurping the spot of an underperforming veteran.
Teams love them because they are cheap, easily replaceable, push veterans or overconfident rookies for there jobs, and round out the roster with youth. A team doesn't have to take any, but usually one or two sneak onto each NFL roster every year.