In some of the modern flex CSS stuff this seems to be a setting governing reactivity of a flexible item. There are really three settings herein so I think...
...is short for:
flex: 1 1 auto;
The three values here from left to right are flex-grow, flex-shrink, and flex-basis. The first two define how an element will shrink and grow relative to others in being flexible and last of these, flex-basis, is the length of an item. The length could be in percentage, pixels, or em or it could have an auto or inherit setting.