Primarily used by police and towing companies, a Denver boot is a clamp placed on a parked vehicle's wheel to prevent it from being driven away. Typically, it is used in retaliation for repeated illegal or otherwise unauthorized parking, in anticipation of receiving delinquent payments of parking fees.
In Darkness Rising, Part 1, Cliffjumper detailed a story to Arcee about how a police officer applied "the boot" to him when he was illegally parked on a stakeout. He waited for them to drive to the end of the block of the street he was on, then transformed out of it, which scared the poor, poor police.
In Impounded, after Bumbleebee and Strongarm were hit by Quillfire's toxin quills during a battle, they woke up to find themselves in an impound lot with boots attached to them, unable to move. The boots proved no hindrance to the bots, as they were easily able to move and transform once Denny and Russell removed their quills.