What is this and what's it for?

A friend gave this to me, and I'm not sure what it's for. I already have a Bluelab pH meter, but aside from water temp I don't know what it does?

It measures TDS (Total Dissolved Solids) and EC (Electrical Conductivity) in a solution. It's how you measure your feed strength.

Ec- electrical conductivity
Tds- total dissolved solids
Temp

It measures temp and Electrical conductivity, or EC, of water.

Pure water is an insulator, in that it does not carry an electrical current. The dissolved solids in the water are conductive. This is used for testing the strength of a solution for hydro or salt fert solutions for soil, or any water source really.