diff --git a/README.md b/README.md
index 7a55f50a..095a8e00 100644
--- a/README.md
+++ b/README.md
@@ -167,7 +167,9 @@ launching weave, e.g.
 
     host1# WEAVE=$(weave launch 10.0.0.1/16 -password wEaVe)
 
-The same password must be specified for all weave peers.
+The same password must be specified for all weave peers; it is the
+basis for creating ephemeral session keys for connections between
+peers.
 
 ### Multi-cloud networking