Copywriting is crafting text for a specific goal (your ad should have a goal in mind)
Good Copywriting:
Allows you to communicate Clearly and Concisely
Emphasize + Focus on your main points
Keeps your audience engaged + Address your audience personally
Easy to read
The best way to appreciate good copy is to look at BAD Copy as shown below 
BadCopywritingismessy:
Content is difficult to read because the jargon used is unnecessarily complicated, beyond the purpose of simple communication and the layout of words are poorly adjusted such as in this case where the paragraph is really long and there isn’t much point for most of the words. Also, who wants to read such a long piece of text?