A cover letter (CL) is required with each submission/round of revision introducing to the paper to the editors about your work in the MS and need to send along with the article.
You need to state the article title and the author(s) of the paper. Journals require their authors to send a CL in the format mentioned in the author guidelines, please check!
In the letter, provide the context, why the findings are important and say why the research is suited for the journal.
What NOT to include in CL
1. Statements that exaggerate or overstate results
2. Conclusion that are not supported by data in MS
3. Cut and paste abstract
4. Too much technical details