This activity goes over the difference between real and virtual images.
When looking through a lens, there are two types of images that you could see. The concave lens should have given you an upside down image, also known as a real image. As light goes through a concave lens, it refracts and converges after exiting. However, it converges underneath the principle axis and looks upside down.
The image you saw through the convex lens is called a virtual image. A virtual image occurs when rays of light enter a lens and diverge after exiting. As a result, it looks like the light is converging at a point where it really isn’t, hence the name virtual image. You can identify a virtual image because it will be upright, look smaller than it is, and look closer to the lens.