Einfach means "simple" and since most adjectives can be used as adverbs - "simply".
Things that are simple (einfach) are often also "easy" (leicht).
It can mean either, and in this context, without a noun, it can't mean "the". I think with time and fluency the evaluation becomes automatic, but I actually kinda think it through as I described.
Every. Time. Oh fluency, you evade me!
Think about how it must feel to somebody learning English to deal with "that".
That is easy - pronoun
That dog is orange - determiner
I saw that you had arrived - conjunction
I'm not that smart - adverb
We fluidly move from one to the other but depending on the learner's language they might be surprised to see us using the same word for all of those!
There are a few on the Internet. Here are my suggestions for approaching it
1) learn about der/die/das as articles. They are equivalent to "the" in English.
2) learn about das as a reference / pronoun, similar to English "that".
It can be confusing but hey, so can English. We can say "That is black" or "That car is black", and "that" fills different roles.
3) Alos read up on "dass" with two s, and when to use it, just to avoid confusion.