Dear Mr.
as I understand "LDAP, Lightweight Directory Access Protocol, is an Internet
protocol that email and other programs use to look up information from a
server." as it's in http://www.gracion.com/server/whatldap.html

ldap is what you said it is but more importantly its just a directory which programs use to authenticate the users accessing that system.
for ex, if you have an ldap directory containing all the usernames of your organization, then you can have many applications looking up for information from it.

i have an openldap server in my institution that is authenticating users for mail servers, data servers, course management software etc.,

i would also like koha to authenticate its users through the same directory just to have the same usernames across all apps.

so why do u need it for koha? could u please explain that to me if you have
time for that