As a precursor another couple of useful misconceptions to discard:
4. Art must be beautiful, or crafted, or at least 'good'. 5. Art has to have meaning.
It's useful to look at the early part of the 20th Century to understand modern art.
A good painting to start this conversation is The Treachery of Images by Magritte[1] from 1929.
On one level it is a simple, acceptable painting of a pipe, with the handwritten label in the painting "This is not a pipe".
What is that label doing? It is in some ways telling the truth, this is not a pipe, it is a painting of a pipe. Already we have opened a useful void between object and reality.
It is also creating a tension in the painting that was not there before. We have an obvious pipe, and we have a counterclaim immediately following. But both the painting of the pipe and the label disclaiming its pipeness are 'true' in that they were put there by the artist and they are unambiguous in their own right.
So the pairing creates an ambiguity that is difficult to resolve. What does the label mean? Do I understand it properly? Is one true and not the other, if so which? What is the sum total of them both? How am I meant to 'read' this, whatever is meant by 'reading'.
Many questions emerge, inhabiting a place outside the painting, in the mind of the viewer. Are these questions part of the artwork? Where does the artwork start and stop? Is my reaction part of it? Why couldn't it just say "This is a pipe"? Is this treachery of some kind? (hence the title of the work).
Once you have seen that this is something a little different to a 'normal' painting, that it in some way occupies a broader space than simply the dimensions of the physical object, then the door of modern art is ajar and cannot be closed again.