A compound noun is a noun that is made up of two or more words. Most compound nouns in English are formed by nouns modified by other nouns or adjectives.

For example:

The words tooth and paste are each nouns in their own right, but if you join them together they form a new word - toothpaste.

The word black is an adjective and board is a noun, but if you join them together they form a new word - blackboard.

In both these example the first word modifies or describes the second word, telling us what kind of object or person it is, or what its purpose is. And the second part identifies the object or person in question.

Compound nouns can also be formed using the following combinations of words:-

Noun + Noun toothpaste
Adjective + Noun monthly ticket
Verb + Noun swimming pool
Preposition + Noun underground
Noun + Verb haircut
Noun + Preposition hanger on
Adjective + Verb dry-cleaning
Preposition + Verb output

The two parts may be written in a number of ways:-

1. Sometimes the two words are joined together.
Example: tooth + paste = toothpaste | bed + room = bedroom

2. Sometimes they are joined using a hyphen.
Example: check-in

3. Sometimes they appear as two separate words.
Example: full moon

There's a list of lots of compound words here.

A good dictionary will tell you how you should write each compound noun.