A keyword is simply what a person types into a search box. Not a magic word you hide in the page, and not something you buy. It is the actual wording of the question, which for a trade is usually a problem and a place: blocked drain Stoke, flat roof leak Leeds, driveway cleaning near me.
Where to find your real ones
Start with your own phone. The words customers use when they ring you are the words they type. People say the gutter is overflowing, not rainwater goods maintenance.
Then look at Google Search Console, which lists the phrases people already used before clicking you. Then type your main service into Google and read the suggestions it offers as you type, and the related searches at the bottom of the results. That is free research and it is Google own data.
Why the obvious one is the wrong target
Roofer is the obvious keyword and it is close to useless. Everyone is chasing it, it is competed for nationally, and the person typing it might be anywhere and might be a student writing an essay.
Roofer in Ossett is a smaller search and a far better one, because the person typing it has a roof and a postcode. Longer phrases like emergency flat roof repair Wakefield bring fewer visitors and more calls, and calls are the thing you are counting.
A worked example
A fencing contractor wants to rank for fencing. Six months of effort, nothing to show, because he is competing with national suppliers and fence panel retailers.
He switches to close board fencing Barnsley, garden fence replacement Penistone and the four other villages he actually works in. Each one is a small search with three or four real competitors, and within a few months he is being found for all of them.