Getting rooted means that your character has been stunned and is unable to move for a moment. Many talents that temporarily ‘root’ enemies in place are AOE, or Area of Effect, which impact an area rather than only a specific hero character (although of course only one hero might be caught in the rooting effect). A notable exception to this is Uther, who has a single-hero stun ability/talent.
‘Rooted’ Gaming Slang Term Worldwide Use Trend