Well, of course "Das ist mir egal" matters. But its meaning is literally: “It doesn’t matter” or “I don’t care.”
The easiest way to start off the explanation is with an example:
A parent is having an argument with his or her child, about, let’s say, eating vegetables. The child doesn’t want to eat broccoli, so the mum or dad eventually starts listing the advantages of the vegetable. “It is healthy,” the parent might say. “You’ll grow to be tall and strong if you eat it.”
The child, however, doesn’t like the idea of eating something green, so his or her answer is always the same: “Das ist mir egal!” The child is saying: “I don’t care!”
“Das ist mir” means something like: “That is to me.”
An important German word
Egal, I would claim to be one of the most important German words to learn. It means “irrelevant,” basically. And you can use it in almost any situation. If you’re telling a story and then realize that your counterpart didn’t listen at all, but asks you to repeat the story, you can just say: “Egal.”
In this case, egal means “It doesn’t matter.” If you’ve got a point of view that you won’t reconsider, but someone urges you to, you can simply say “Egal was du jetzt sagst, du kannst mich nicht umstimmen.” You are telling them: “It doesn’t matter what you say next, you cant change my mind.”
Now that we have established the meaning, let’s get into the history of the word egal. The expression actually comes from the French word égal, which in turn, goes back to the Latin word aequalis. Both of these words mean “equal” or “even.” Up to the 19th century, egal used to mean “equal” or “the same” in German as well. But since then, its meaning changed from “equal” to “indifferent.”
It hasn’t lost its original meaning completely, though. If you look at synonyms for: "Das ist mir egal", you will find "Das ist mir gleich" (“It’s all the same to me.”) However, you’ll probably find more people using the word egal than gleich – it has established itself firmly in the German language.
Egal has a superlative too, by the way, and I am not talking about egal, egaler, or am egalsten (which is grammatically correct.) I am talking about scheißegal, a nod to the German language's tendency to stray into vulgar territory.
Use that if you want to tell people that you really, really don’t care.
Das ist mir scheißegal.
I really don’t care/I don’t give a f***.
Egal was wir machen, ich bin dabei.
I’m in, no matter what we’re going to do.
Es ist mir so was von egal.
It really doesn’t matter at all.