Certified GKH Gardening Expert
This is, simply, the water pushing out the air in the soil as it filters down to the bottom of the container. This will be more pronounced if there are no drain holes at the bottom, as the air will all have to escape out of the top. There is nothing to worry about here.
This article will help you with the care of pothos: https://www.gardeningknowhow.com/houseplants/pothos/pothos-plant-care.htm