diff --git a/README.md b/README.md
index d3e995e..6031177 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,8 @@
 # magic-wormhole-transit-relay
 Transit Relay server for Magic-Wormhole
+
+This repo implements the Magic-Wormhole "Transit Relay", a server that helps
+clients establish bulk-data transit connections even when both are behind NAT
+boxes. Each side makes a TCP connection to this server and presents a
+handshake. Two connections with identical handshakes are glued together,
+allowing them to pretend they have a direct connection.