How would you briefly explain it without a full length discussion?
Explaining anything religious to an atheist is inevitably a frustrating experience, regardless of what the topic is. As I see it, there are three ways of dealing with atheists’ questions:
The first option is to present an answer in a way that they can swallow, which basically means you’ll have to talk about it as if your religion is just your funny little hobby. The advantage of this approach is that you’ll avoid awkwardness, discussion, etc. The drawback is that you’re being insincere, and in a subtle way this is unhealthy for your own spirituality.
The second option is to be dead serious about it. The atheist will not like this. He/she will get uncomfortable and will usually respond with comments or questions that are either explicitly or implicitly defiant. Even if you don’t get into a full-blown argument, the atheist will still remember you as a religious person that they don’t particularly care for. The disadvantage is obvious: you won’t be making friends this way. There is also an advantage however: you will maintain full integrity for yourself, and you won’t have to live with the feeling that you’re selling your religion short whenever an atheist comes poking his nose.
The third option is to dismiss the question with a short answer like “It’s a religious thing; I prefer not to talk about it.” The atheist will likely still challenge you, saying something like “Well I’m just asking, y’know, no need to be so secretive about it.” But that you can dismiss with something like “We don’t usually discuss religion outside our community.” This avoids both the awkwardness of a discussion as well as the insincerity that is required to have a “nice” talk about it.
P.S. Note that there are two kinds of atheists: disinterested atheists and aggressive atheists. The former just don’t care either way, and typically don’t ask questions about matters religious. When they do, it doesn’t matter much how you answer. To them it’s just chit-chat. The aggressive atheists, however, are actively anti-religious, and unfortunately it is often them that ask most questions about religious matters.