A breakpoint is a predetermined screen size that has specific layout requirements. At a given breakpoint range, the layout adjusts to suit the screen size and orientation. Designers used to look at specific device breakpoints, such as an iPhone X or an iPad, but nowadays it's preferred to make a responsive design and use as less breakpoints as possible since that means less work for developers. It's efficient to start with your smallest design, often a width of 320px, see when it starts to looks shitty and then decide what breakpoints you need.
I made this website responsive, so now we only need three breakpoints.
See also: Mobile First