A WordPress theme dictates the overall design and layout of your website. It decides which fonts you’ll use as well as their size and color, background color, etc.
Anything on your screen that has a size, color, and particular position is all covered in the theme.
There are two main kinds of themes: commercial themes and custom themes. Yes, there is a third category called “free themes.” While there are some quality free themes out there, as a general rule, you get what you pay for.
A commercial theme is one that you can purchase from a theme store, of which there are many. A custom theme is one that is completely custom built for you.