Hola John

Neutral article "lo" can mean different things depending on how it is used; there isn't one single translation for it. In this exercise we are using these two sentences: 

1. "...porque no llovió lo suficiente."

In this case, "lo suficiente" functions as adverb "suficientemente" (sufficiently/enough). It is also used sometimes with "bastante", also meaning "enough", for example: "Yo gano lo bastante para sobrevivir." (I earn enough to get by/survive.) 

2. Nuestro planeta se está calentando más de lo normal." (Our planet is getting warmer than usual.)

 In this case, we mean "more than what it is the usual thing." This is a set structure: "más de lo +adjective" / "menos de lo +adjective" Here is another example with a different adjective: "Nos pagaron más de lo acordado." (They paid us more than what was arranged.) 

"Nos pagaron menos de lo acordado."  (They paid us less than what was arranged.) 

You can have a look at some information about neutral article "lo" and some of the cases we've covered in some lessons: 

Artículo neutro "lo"

I hope this helps,

Un saludo

Inma