Breakpoints are adjustable widths that affect how your layout responds across device sizes, and allow adapting at a specific viewport.
Breakpoints are the values of pixels that are defined through the use of CSS. The website will adapt when it reaches a certain size value and lower/higher.
There are multiple types of breakpoints, some common ones include:
A developer will decide which kind of breakpoint to use, based on what devices they had decided to make their website relevant to. Most internet users are mobile devices, so most developers have adapted and almost all modern day websites must contain mobile compatibility, or potentially face obscurity.