Influenza hemagglutinin (HA) is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected. HA proteins bind to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes. [1]
HA has two functions. Firstly, it allows the recognition of target vertebrate cells, accomplished through the binding to these cells' sialic acid-containing receptors. Secondly, once bound it facilitates the entry of the viral genome into the target cells by causing the fusion of host endosomal membrane with the viral membrane. [2]
Since hemagglutinin is the major surface protein of the influenza A virus and is essential to the entry process, it is the primary target of neutralizing antibodies.

References:
1. Russell RJ, Kerry PS, Stevens DJ, Steinhauer DA, Martin SR, Gamblin SJ, Skehel JJ (November 2008). "Structure of influenza hemagglutinin in complex with an inhibitor of membrane fusion". Proc. Natl. Acad. Sci. U.S.A. 105 (46): 17736–41.
2. White JM, Hoffman LR, Arevalo JH, et al. (1997). "Attachment and entry of influenza virus into host cells. Pivotal roles of hemagglutinin". In Chiu W, Burnett RM, Garcea RL. Structural Biology of Viruses.OxfordUniversity Press. pp. 80–104.