Meaning of 1,1,1,2 in 1,1,1,2-Tetrafluoroethane & similar
The numbers indicate which carbons the substituents - the fluorine atoms - are on. The root of the molecular name is "tetrafluoro," indicating the presence of four fluorine substituents. So now where are they? That's what the numbers tell us; the "1,1,1" part indicates that there are three fluorine substituents on the first carbon, and "2" tells us there is one more fluorine substituent on the second carbon. 

Which is the first carbon and which is the second carbon? The numbering scheme should be selected such that it gives the lowest number at the first point of difference. 

This way, we can write "1,1,1,2" to indicate the presence of the four fluorine substituents rather than "1,2,2,2."