Coffee is made of 98% water. Great coffee needs great water to begin with.
The sources will determine the mineral content of the water; with the ability to control the levels, it helps to improve the flavours you can extract from your cup. The ideal water for brewing boils down to the amount of minerals in your water. Too little, you do not have enough solvents to extract flavours out from your coffee. Too much, you are also limiting the amount of soluble compounds you can extract from your coffee.
A TDS meter is a convenient digital gadget to get instant measurements on your overall water softness. With a push of button, it will show you the Total Dissolved Solids (TDS) level, or the mineral level of your water.