Your cover letter allows you to talk about yourself and your good qualities in a more ‘conversational’ way, rather than just lists and bullet points as your CV might be.
You should include brief information about:
Your qualities e.g perseverance (include examples too to prove this)
What you can bring to the company
Why you want to work for the company
Some of the things you say will also be on your CV, but the cover letter gives you a bit more opportunity to elaborate on this.