Certificate checking is used by ESXi hosts to check each others identities, which helps prevent man-in-the-middle attacks on the host. The default, self signed, certificates generated by an ESXi host are vulnerable to these attacks however, so certificate checking works best alongside certificates assigned from a valid internal certificate authority.
Enabling/Disabling Certificate Checking
You can enable/disable certificate checking in the vSphere client, when connected to vCenter. When logged in, select Administration then vCenter Server Settings:
