Keywords (in terms of SEO) are words or phrases typed into search engines by the user to find a relevant website. In order to help your website be discovered, it's important to target the keywords your visitors are likely to use.
If I was looking to buy a new pet locally, I might search for “Pet shops Brighton”.
Or if I was looking for a recipe I might search for “Spaghetti Bolognese Recipe”.
Knowing which keywords to target is one of the toughest parts of optimising your content. Target keywords that are too general and you may not rank highly enough; target keywords that are too obscure and no one may ever search them.
In general, you should start by focusing long-tail keywords - these are a few words that may be searched together. If you sell furniture, trying to optimise for "chairs" means a lot of competition, but you may have a better chance with "handmade antique wooden chairs."
To begin researching, simply think of some words or phrases you'd like to optimise for and put them into a search engine. Is the competition too stiff? Why not look at competitors and see what they're doing to help get ideas and see what you're up against.
Once you've got your keywords and added them to your content, you can begin measuring the impact with tools such as Google Analytics. It's important to constantly monitor your performance and looking for extra opportunities to boost your ranking.