Sentence case capitalises the first letter of each sentence and lowercases the rest, apart from words that need capitals of their own.
Where it is the right style
Most modern interfaces and publications use it for headings. It reads as more natural and less shouty than title case, and it is markedly easier to apply consistently across a large set of headlines.
The usual input
Content arriving from multiple writers with no house style — some in title case, some in all caps, some in whatever the CMS did to it. Normalising to one convention makes the set look deliberate.
The one thing to check afterwards
Proper nouns and acronyms. Automated conversion lowercases them along with everything else, so brand names and initialisms need a pass by eye.
Try it: Convert Text to Sentence Case on SeoWolf's Notepad.