Party Whips in Congress are groups that assist the floor leaders of the party they belong to, that make sure that their floor leaders (or legislators) attend voting sessions and that impose discipline and direction to them in order to secure enough votes on measures that are congruent with the party's platform. This way, party whips guarantee that legislators do not vote according to their ideologies, but according to the party's ideology.
Therefore, Party Whips influence legislation by mobilizing votes within their parties on any legislation and by ensuring that legislators vote according to the party's desired measures.