A useful summary does three things. It states the type of role the candidate is targeting, gives one or two concrete signals of relevant experience, and adds one detail that is not obvious from the rest of the resume. Anything else is filler.
Consider a software engineer applying for a platform role. A weak summary reads: "Passionate software engineer with strong problem-solving skills and a proven track record in fast-paced environments." A stronger summary reads: "Backend engineer with four years building platform services in Python and Go. Shipped the migration from a monolithic deploy to containerised services in under a month." The second version tells the recruiter what the candidate actually does and how.
If you are early in your career and your experience section is thin, the summary can carry more weight. Describe what you studied, what you built, and what type of role you are targeting. Be specific and stay honest — a recruiter will find out quickly if the summary overstates your experience.
Keep the summary under four sentences. Two or three is typical. If you cannot fill three sentences without repeating the experience section, cut the summary and give the space to the experience section itself.
Match the summary to the role you are applying for. The same candidate may want a different summary for an IC role, a management role, and a career-change role. Save the versions as separate drafts so you can iterate without retyping.
The last thing to avoid: do not begin the summary with "I am". Resumes are not written in the first person. Just state the facts.