CalMag is a mix of calcium and magnesium.  A great base nutrient containing all the essential natural buffers that your nutrient solution needs to maintain stable pH and EC readings. It is also an essential additive for heavy fruiting crops such as tomatoes and peppers.  It also helps to prevent blossom end rot in tomatoes.
 
Alex's Top Tip: if your EC is below 0.4 before adding any nutrients, I reccomend adding CalMag until the EC reads 0.4 before adding your regular base nutrients in the normal dilution ratio.