A theme is said to be responsive (adaptive, reactive) when it is designed to provide a site visitor with an optimal viewing experience that facilitates reading and navigation regardless of the device used (computer, tablet, smartphone).
The site display automatically adapts to the screen size and resolution. This avoids having texts that are too small when consulting a site on a phone for example.
An increasing number of people are accessing the internet using tablets and smartphones. This is why it has become crucial to choose a responsive theme for your site, otherwise you risk being penalized by scaring away many visitors.
In addition, it is important to know that Google highlights responsive sites on its search engine.
To find out if your site is responsive, resize your web browser window and see if the elements reorganize themselves as the space gets smaller. You can also use this site to test.