What it is, without jargon
Your website says things in a way people understand: an address in a footer, hours on a contact page, prices in a table. A machine reading that has to guess which number is a phone number and which is a price.
Structured data is a hidden block of text on the page that states those facts explicitly, in a standard format. No guessing required. That is the whole idea.
The three that matter
LocalBusiness: who you are, where, hours, phone, service area, price range. This is the foundation and every local site should have it.
Service: what you actually do, described one service at a time, tied to the areas you serve. This is what lets a machine match you to a specific need in a specific place.
FAQPage: your genuine common questions and answers. Useful because answer engines lift directly from clearly structured question-and-answer content.
What it does and does not do
It does make you eligible for richer search presentation and easier for answer engines to cite. It does remove ambiguity that causes omission.
It does not improve rankings by itself, and it will not rescue a site with no reviews, no profile, and no relevant pages. It is a multiplier on fundamentals, not a substitute.
Getting it done
Most site platforms have a plugin or setting for it. If yours does not, it is a small block of code added once per page type, not an ongoing project.
Then validate it — free testing tools will tell you whether it parses and what a machine sees. Invalid markup is common and silently useless, so the validation step is not optional.