Logrolling is the exchange of favors, such as trading votes to gain passage of actions in the legislature. Basically, Politician A would be obligated to vote for Politician B’s legislation if B agreed to vote for A’s as well. It’s a simple deal – usually involving earmarked legislation for their own districts.
The term “logrolling” came from a lumberjack sport that involves standing on and spinning a log that is floating on the river. The goal is to try and cooperate with the log or to throw off another competitor.
If the politicians agree to roll the log together, then they help each other out and no one will “fall off the log.”