Is

'Is' further describes something. 'Are' is used for multiples.

Here are a few examples of the use of the word 'is' - there are more uses than these.

If I say, 'The rock is grey.', then I am explaining that a particular rock has a grey color.
If I say, 'Light is everything.', then I am explaining that light composes everything.
If I say, 'A duck is a bird.', then I am explaining that all ducks belong to a larger group named birds.