IP Authentication Header (IP protocol 51) is used to provide connectionless integrity and data origin authentication for IP datagrams, and to provide protection against replays.
In plain English this means that AH protects packets against tampering but it does not provide encryption: the payload of the packets is not kept confidential. AH packets are in plain text. In practice, almost nobody uses AH.
IPsec AH is defined in RFC 2402.
