A compound noun is two nouns pushed together to make one word. Usually the two words become one word without a space between. When we leave a space in a compound word, we call that an open compound noun. The two nouns go together to make one compound word, but we write the word with a space between the two nouns.
Here are some open compound nouns:
cell phone
high school
video game
candy cane
These compound nouns are made up of two nouns together. Even though we leave a space between the two nouns, the two nouns act together as one word. Saying just candy or saying just cane would not mean the same thing as saying candy cane. These two nouns together say the name for one object, so these two nouns together are a compound noun.
Here are some more open compound nouns:
fire drill
coffee mug
vice president
swimming pool
hot dog
When two nouns together make a separate word, we call that word an open compound noun.