SEO is an acronym for Search Engine Optimization - it is the process of improving a website to rank better in the search engines. It's comprised of on-site and off-site actions.
On-site includes things like content and page elements, URL structure, internal linking, schema markup, site speed, etc.
Off-site includes things like backlinks to your website, citations, etc.
This is difficult to answer accurately because every business is different. Some business types are highly competitive while others aren't as much.
A good rule of thumb is 3 to 6 months before you'll start seeing improved results. But it could be as little as one month for local businesses and low competition areas.