It means that you have a bad cell signal or telephone connection, causing your words to come in reception and out of reception, like this: "If we...having...conversa...on...then...other side...might...get..ev..y...othr..wo" (If we were having a conversation, then the person on the other side might get every other word.)

Or, potentially, the car in front of you is weaving in & out of the lane in front of you. Thought you rarely get the chance to tell them this, so I think Joe's explanation is entirely more likely. . It doesn't have to be a cellular call. You could use it for someone who keeps coming & going from a real life face to face conversation.