In this sentence "Apple is red.", what does the word "apple" refer to? Does it refer to "all apples" or "each?

First of all it is NOT an actual sentence.

"The apple is red", is the correct way to write that sentence.

When adding the article "the", as in The apple.. It refers to THAT apple.

If the sentence was written: ALL apples are red. Then the statement suggests ALL apples are indeed red, which of course is not true.

One could also write: An apple is red. This also suggests ALL apples are red.

..that's why you cannot believe everything you read/hear.

"Apple is red." is not a sentence. If you wanted to refer to one apple you would use "The apple is red: and if you want to refer to all apples you use "An apple is red."

Uhhhh, "apple is red"is not a sentence. It is jibberish, so I guess apple refers to nothing

It's a name of someone/thing.