We use a superlative adjective to describe one thing in a group of three or more things. Look at these examples:

1. Shyam is 5 Feet. Arun is 6 Feet. Sohan is 4 Feet. Arun is the tallest.
2. Canada, China and Russia are big countries. But Russia is the biggest.
3. Mount Everest is the highest mountain in the world.

If we talk about the three planets Earth, Mars and Jupiter, we can use superlative adjectives
1. Jupiter is the most distant from the Sun.
2. Jupiter has the most moons.