First, turn on your imaginations and let's pretend that we are crystals of salt floating around in the fresh water.
Fresh water is water that has very little salt in it.
Fresh water comes from the evaporation of the seas and precipitation, which makes it a continually renewable resource. As we flow into the ocean, we are being dissolved. This is because there is hardly any salt in the river water. The little amount of salt that is carried into the ocean stays in the ocean and is concentrated over time, while the water that is in the ocean evaporates. As more salt is carried into the ocean, the ocean gets saltier and saltier.