diff --git a/nixos/lib/make-channel.nix b/nixos/lib/make-channel.nix
index fd805f7f943..0db4a46151d 100644
--- a/nixos/lib/make-channel.nix
+++ b/nixos/lib/make-channel.nix
@@ -1,3 +1,7 @@
+/* Build a channel tarball. These contain, in addition to the nixpkgs
+ * expressions themselves, files that indicate the version of nixpkgs
+ * that they represent. This only seems to be used by Travis.
+ */
 { pkgs, nixpkgs, version, versionSuffix }:
 
 pkgs.releaseTools.makeSourceTarball {