Lazy loading means a page does not download every image the moment somebody opens it. Images further down wait until the visitor scrolls near them. The first screen appears fast, and the rest arrives as it is needed.
Why a trade site needs it
A good trade website is mostly photos. A gallery of forty jobs, before and after pairs, a team shot, a van. Loaded all at once on a phone that is on two bars in a village, that is a page that keeps a customer waiting long past the point most of them will give you.
With lazy loading the visitor gets your first screen almost immediately and never notices the rest arriving. Nothing is removed from the page. It just stops competing for the same slice of signal.
The one image never to lazy load
Your main hero image at the top of the page must load normally. Telling the browser to wait for it delays the largest thing on the first screen, which is exactly the measurement Google uses for loading speed.
So the rule is simple. Everything below the first screen gets loading=lazy. The hero does not, and is better off marked as high priority so it starts downloading first. Google explains the trade off in its browser level lazy loading guide.
A worked example
A builder with a hundred and nineteen gallery photos across five tabs would be asking a phone to pull the lot on arrival. With lazy loading only the visible handful load, and the rest come in as the tab is opened and scrolled.
The homeowner gets a page that feels instant and still sees every photo when they want to. The firm keeps the gallery that sells the work.