It's quite brief. We state that all the authors agree to submit the paper to the Journal. We state that the work is not sent to another journal, that it's our own work (certainly not plagiarized). We state that we would be grateful if the article is considered for publication in that Journal.
Some journals give us an example of a cover letter.
After I sent the email with the cover letter attachment, and my softcopy manuscript, I got an email that the submission was a complete submission and that they would give me a submission number, within 10 days.
The cover letter provides an opportunity to explain to the journal editor how your research is innovative and why your manuscript should be published in the particular journal. A clear, direct and interesting cover letter can increase the chances of a journal editor sending your manuscript for peer review. That is why it is worth spending time writing a coherent and persuasive cover letter.