When we gather our resources together, we say "to pool our resources". So to me, "carpool" is where several people have pooled their resources (cars) and agreed to use one car for several people (ie. on a roster) instead of each person using their own car. So individual petrol costs are cut down to a quarter or even a fifth.

我理解就是拼车啦。

Meaning of car pool as a noun 1. An arrangement whereby several participants or their children travel together in one vehicle, the participants sharing the costs and often taking turns as the driver. 2. A group, as of commuters or parents, participating in a carpool. As a verb, 'carpool ' means to travel in one car