What are they looking for
The machine
The HR Candidate system that you submitted your resume to online is sorting through the resumes submitted for a particular job looking for the keywords that are found in the job description. The number of keyword matches and the frequency of the matching of individual keywords will be identified and resumes will be sorted in order of "hits."
An administrative staff person
The administrative staff person who reviews the results of the keyword search is scanning through the results looking for readability, length of employment in recent roles and appropriateness of job titles.
The Human Resources Manager
The HR Manager will review the 15 or 20 resumes from the staff person t find the best 10 or 12 based on job title, industry experience, number of people managed, etc. as well as any other criteria that the company's experience has shown makes for a good fit both culturally and technically. This is generally more subjective and anecdotal than prior steps so researching the company and presenting yourself as a good fit now becomes critical to getting to the next step.
The hiring manager
The hiring manager will look for the same criteria as the HR Manager with respect to fit and will also read into the details of your experience with their pain points in mind to see who is worth talking to.