What it's for
The summary answers the question a reader forms in the first second: what is this person, and why am I looking at them for this role? It is orientation, not persuasion. Get that right and the rest of the page is read in the correct frame.
It is not an objective statement. Nobody needs to be told you are seeking a challenging position with a forward-thinking company.
A formula that holds up
Sentence one: what you are and for how long, with the domain. Sentence two: the ground you cover, or your strongest evidence. Optional sentence three: what you are pointed at now.
Every clause should be checkable against the rest of the resume. If the summary claims something the experience doesn't demonstrate, you have written a cover letter by accident.
Worked examples
Software engineer: "Backend engineer with nine years on payments and identity systems in regulated fintech. Owned a ledger handling 2M transactions a month and cut authorisation latency from 800ms to under 200ms. Now looking for staff-level work on high-throughput infrastructure."
Product designer: "Product designer working on complex B2B tools — the kind with permission models and audit trails. Led the design system that took the company's four inconsistent products onto one component library. Strongest where research and interface work meet."
Career changer: "Secondary school teacher moving into instructional design, with a postgraduate certificate completed alongside four years of classroom work. Built the department's curriculum materials for 200 students and trained six colleagues on delivering them."
What to cut
Adjectives about yourself. "Detail-oriented", "passionate", "hard-working" and "results-driven" are claims nobody can verify and everybody makes, so they read as noise.
Also cut the first person. "I am a backend engineer" wastes two words; resumes are written in an implied first person and every reader understands it.