c its very simple

D - mean Dextro (right) rotation

L - means leavo ( left) rotatory

 

so wen v r given an optically active or inactive compound having a chiral centre , then v can check da rotation made by da compound. if it is towards left or right

 

we 'll give da preferences acc to da At. wt of da first grp attached to it

 

Basically it is frm H to OH

 

Like for eg in lactic acid