You maybe asking what are entities well they are a fancy word for mobs such as zombie, pigment, skeleton, wolf, pig and much more. Some entitys can be a bat such as zombie skeleton and more but some are good like cows and pigs you can’t summon entities are using a command go to that project entities could also be item frames, total mass of him dying, Ender pearl, Eye of Ender, boats,
and more items. Entities could also be stuff that is on the ground not placed but broke it such as dirt, cobblestone, and more thanks to wethebean for updating me