If a whole website is a library, on-page SEO is about labeling one single book well: its cover, its chapter titles, and the summary printed on the back. These are things you can change inside one page's own code. Here is where each one lives.
Title tag
The title tag is the big, clickable blue line you see in a search result. It is also the text shown on the browser tab. Give every page its own clear title.
Meta description
Right under the title in a search result, there is often a short line of text called the meta description. Search engines may show it, or swap in different text instead. It does not directly change your rank, but a good one can make someone want to click.
Headings
Headings, like <h1> and <h2>, work like a book's chapter titles: they show what the whole page is about, and what each smaller section covers. Clear headings help a reader skim, and they help a search engine understand the page's structure too.
Internal links
Internal links are links from one of your own pages to another one of your own pages. They help a visitor — and a crawler — find more of your pages, and they show which pages are related to each other.
Alt text for images
Alt text is a short description written into an image's own code. A visitor usually never sees the text itself, but a search engine reads it to understand the picture, and a screen reader speaks it aloud for someone who cannot see the image.