In the example “En esta casa entra muy poca luz.” please explain the use of “entra”.

Hola Johan

In this sentence:

We are using the verb "entrar" (to enter/to get in) because when we talk about light we sometimes use it in order to express how much light there is in a house or how much light "gets in" the house. It is something I believe would sound odd in English but it is used in Spanish. 

You could also use "hay" with pretty much the same meaning:

We can also use it to say that there is a lot of dust in a house:

I hope it helps,

Inma