Typically, employers look at a resume first to make sure you have the desired skills and experience before taking the time to read your cover letter and get to know you a bit more. This is especially true in fields that require specific hard skills or specific qualifications, such as IT, architecture and engineering.
However, in more creative fields like marketing, sales and copywriting, employers may read your cover letter first to ensure that your writing skills are at the level they require.
It is also important to note that if a hiring manager or employer asks specifically for you to include a cover letter with your resume, and you didn’t – it is highly likely they won’t bother reading your resume at all.