What does write properly or don't write at all mean?

If you cannot write correctly, then you are not able to communicate by writing correctly. So it is better to not write all , in order not to confuse the reader.

Learn to write correctly, by forming the letters correctly. Don't allow 'a' or 'o' etc., to remain 'open' ; this makes the letter look like a 'u'. There are many other examples.

To aid correct writing, use a fountain pen, or a dip nib pen, or even a goose quill. Modern biros allow the hand to 'slip' and there by not correctly form the letters.