This chapter is about entities, so lets start with describing the definition entity. An entity is an object which interacts with the game in any way. Until now, we have already encountered two different entities, first, the light entity which simply specifies a point in the map where light should emit.
Second, the info_player_spawn entity which tells the game where to place the player when he gets into the game. Both of these are entities and they are very different, and both of them just define points in space. There are also entities which have to be linked to an object to function, a door for example, here the geometric object representing the door is also a part of the entity, but more on that later.
Entities always have certain properties you can give to them, for example on a door, you would like to specify how fast it opens, in what direction it should open and so on. And for something as simple as a light, you would want to specify the color as well as how strong it is, if it should flicker or not etc. etc. All of this and a bit more will be covered by this chapter.