Thumb pads were a primary form of entry into a building or room. Rather than a doorknob which had to be turned and locked, a thumb pad identified the user by their unique thumbprint and welcomed the person once they passed security.
However, thumb bandits were known to steal people's thumbs to use them to gain access to secret or private areas.
AT&T were one supplier of thumb pads.
Because Jennifer Parker's thumbprint never changed over the years, the Hill Valley Police assumed she was the Jennifer of the future and she was able to enter her home.