The MLSS is a measurement of ...

The MLSS is a measurement of the concentration of the suspended solids in the aeration tank.  The SVI is a measure of how well the suspended solids settle and compact when left undisturbed for 30 minutes, except that the lower the number the better it has settled since it is expressed in units of ml per gram.

I looked in an old textbook of mine to refresh my memory of these terms ("Unit Operations and Processes in Environmental Engineering" by Tom D. Reynolds, published by PWS-KENT in 1982).  The glossary provided these two definitions.

• Mixed Liquor Suspended Solids (MLSS): The suspended solids in the mixed liquor- that is, the mixture of wastewater and activated sludge undergoing aeration at an activated sludge wastewater treatment plant.
• Sludge Volume Index (SVI): The volume in millilitres occupied by one gram of mixed liquor after settling for 30 minutes in a litre graduated cylinder.

The book also gave this overview of how the two measurements are used to regulate the operation of the plant's recycle ratio.  See attached PDF.