On-page SEO covers everything on a webpage itself that affects how it ranks — as opposed to technical SEO, which deals with how a site is built and crawled. Think of it as making sure every page clearly tells Google, and your visitor, what it's about, in a way that's easy to read and easy to trust.
For a business website, this often means the difference between a page that gets found for the right searches and one that gets overlooked, even if the underlying business offer is strong. A services page titled simply “Home” or “Services” tells Google very little. A page titled around what the business actually does, with headings that follow a clear structure and content that answers real questions, gives Google much more to work with — and gives your visitor a reason to stay.
On-page SEO isn't a one-time checklist. Every new page you publish either supports your visibility in search or works against it, so getting the fundamentals right early saves you from rebuilding pages later.