When we talk about focus in linguistics, we mean the most important part of a sentence or conversation. Imagine you're telling your friend a story about your day. If you say "I went to the store," that's just a boring fact. But if you say "I went to the store and I saw a puppy!" then the puppy is the important part, the focus.
You can use different words or ways of saying things to put different things into focus. For example, if you say "I ate pizza for dinner," that's just a fact. But if you say "It was pizza that I ate for dinner," then pizza is the focus because you're emphasizing that it was specifically pizza that you ate.
We use focus to make sure that people understand the most important parts of what we're saying. Sometimes we use it to compare things or to help someone understand why something is important. It's kind of like using a highlighter to make sure people notice the most important parts of what we're saying.