When we are out on the sun light our pupils narrows to avoid the entrance of too much sun light. So when we go inside, since our pupils are still narrow... it will be dark

Because when we are out in the sun our pupils constrict to decrease the amount of light entering our eye. So when we enter somewhere relatively dark until our pupils dilate to adapt to the smaller amount of light less light is entering our eye which makes the room more darker

Min agebah

At that moment your eyes were used to the high amount lighting from the sun then when you instantly walk into a shade your eyes needs some time to adapt the environment

It's called accomodation.