Sentence:  If a device's screen width is equal to, or less than, the screen width defined at the <WSD>breakpoint</WSD>, it will have the related CSS applied to it. You sometimes want to set breakpoints to match particular device screen widths, but I have found that what is most important is simply that the layout remains usable as it gets smaller. 
Meaning of the word marked with <WSD> tags:  A particular size at which a web page is designed to switch to a different layout.