A responsive theme has a fluid layout that automatically adjusts to the various screen sizes and resolutions of electronic devices. For example, they may shrink, expand, or change the layout of components to optimize how the page appears on a laptop, tablet, or mobile phone. They account for the various possibilities in a single theme, and eliminate the need to use different themes or designs for Web vs. mobile display.
The WordPress Theme Directory offers a wide variety of responsive themes. You can find them by searching the directory for the word “responsive.”