Keyword research is finding the words your future customers type into a search box, so your pages use the same words.
A keyword is a word or phrase a person types into a search engine. Keyword research is the habit of finding out which ones your customers actually use, how often, and which you have a realistic chance of ranking for. It matters because people search in their own words, not in yours. A plumber may write "hydronic heating service" while customers search "radiator not heating".
A page that uses the words of a search is easy for a search engine to match to it, and easy for the person to recognise as the answer. Pages written without that check tend to be about what the business finds interesting, which is rarely what brings in enquiries.
List the problems you solve. Write each as a customer would say it to a friend. Include the products, places and prices people ask about.
Ask real people. Read your sales emails, call notes and reviews. The phrases customers repeat are your seed list.
Type each phrase into a search box. The suggestions that appear as you type, and the "related searches" at the bottom of the results, are searches other people made.
Look at who ranks. If the first page is full of large publishers for a broad phrase, that phrase is hard. Narrower phrases, see long-tail keywords, are usually a better start.
Check your own data. If you use Google Search Console, its performance report lists the queries that already show your site. Pages sitting at positions 8 to 20 are often the cheapest to improve.
Put phrases that mean the same thing together, then give each group one page. "Emergency plumber", "24 hour plumber" and "plumber open now" want one page, not three. If you give two pages the same group they compete, see keyword cannibalization.