What does it mean when you dream of riding the bus?

There is not enough information to provide a reliable interpretation. If the dreamer sees the father leaving on the bus, with the dreamer left behind, it suggests that the dreamer is beginning to accept the reality of the father's death and is making progress through grief. If your father is long dead, and you have long since made this peace, but dream of your father on a bus, he is headed somewhere - or returning from somewhere, or going away from somewhere. The significance of that to you is something that requires thought on your part. The answer is in your subconscious - or you wouldn't have dreamed it. Think about it at bedtime. It is likely the answer will come to you in a dream vivid enough to actually wake you up.