Answer and explanation:
A metaphor is a figure of speech used to compare two things that are very different from each other. The intention of such comparison is to attribute a certain quality of one thing to the other by stating something of the sort "thing A is thing B". By the way, that is what differentiates a metaphor from a simile. Even though both figures of speech are used to compare things, a simile depends on support word - as / like -, while a metaphor doesn't. An example of a metaphor would be saying, "your smile is a rose". The intention would be to confer one or more qualities of a rose to someone's smile, such as the softness of the petals to the lips.
By stating that planet Earth is a dot in space, the speaker's intention is to confer the quality of a dot to our planet. A dot is something small and insignificant. If our planet is a dot in space, that means it is small and insignificant when compared to the majesty of the whole universe. That perspective can be unfolded into broader meanings, such as the triviality of our daily lives, our problems that seem so big to us, but are nothing to the cosmos. If we live or die, the universe itself does not cease to exist.