Authentication Header - IPSec protocol
IPSec uses two basic protocols, AH (authentication header) and ESP (encapsulation security payload). AH ensures data has not been tampered with and assures data integrity when in transmission. This is achieved by adding authentication information to a datagram. AH is not as used much as ESP as it does not provide data encryption (confidentiality) and so all data would be transported in clear text. So data would be readable although protected from any modification attempts.
However if authentication is all that is required then only AH should be used. By leaving ESP turned off will provide better performance.
Wikipedia's guide to IPSec