A simple example of positive buoyancy: place a piece of wood on water. Wood is less dense than water, therefore it floats. Buoyancy counteracts the weight and the net force on the piece of wood is zero.
If you submerge a block of wood below the surface and release it, it will buoyantly rise to the surface. Buoyancy is larger than the piece’s weight, therefore there is an upward net force which makes the wood rise.