A compound noun is formed from an adjectif and a noun, or two nouns.
Compound nouns are usually written as two words,
example: mother tongue.
But sometimes they are jointed by a hyphen,
or written as one word,
unfortunately, there is no rule for this.
So let's find others compound nouns and try to give their meaning.
I'll start with those giving as examples:
mother tongue: 1st language
baby-sitter : a person who looks after children when their parents are absent
toothpaste: a substance we use to clean our teeth[