diff --git a/cosmic_ray/interceptors/__init__.py b/cosmic_ray/interceptors/__init__.py
index <HASH>..<HASH> 100644
--- a/cosmic_ray/interceptors/__init__.py
+++ b/cosmic_ray/interceptors/__init__.py
@@ -0,0 +1,5 @@
+# An interceptor is a callable that is called at the end of the "init" command.
+#
+# Interceptors are passed the initialized WorkDB, and they are able to do
+# things like mark certain mutations as skipped (i.e. so that they are never
+# performed).
Added docstring describing what an "interceptor" is.