It's the amount of salt in your water. It's a comparison of the weight ratio of the water with salt dissolved in it.
Distilled water's specific gravitiy is 1.00. Seawater at 1.025 has enough salt in it to weight 1.025 times as much as distilled water.
People usually use the term "specific gravity" and the term "salinity" interchangeably, so maybe that's the term you are familiar with. Nonetheless, "specific gravity" is the proper term for the 1.0xx measurement, as salinity is measured in parts per million, and nobody ever really measures it that way.