If you set the "User Base DN" to your LDAP Root DN, it means that the search will start from the Root DN until it finds all the results that matched the search filter.

So, setting the "User Base DN" precisely is very important as it will decide where the search is starting from. It will save all the unnecessary search between the Root DN to your "User Base DN".

DC=joget,DC=org

DC=HR,DC=joget,DC=org
DC=Product Department,DC=joget,DC=org
DC=Operation,DC=joget,DC=org
DC=Users,DC=joget,DC=org

If your users are all under "DC=Users, DC=joget,DC=org", you should set this to "User Base DN".
By doing this, it will not go through all the other entries and it's child entries before reaching "DC=Users, DC=joget,DC=org".

(&(objectClass=person)(|(cn=admin)(cn=cat)(cn=jack)(cn=john)(cn=jackie)))

This mean all the LDAP entries which have "objectClass" attribute equals to "person" and "cn" attribute equals to either "admin", "cat", "jack", "john" or "jackie" are Joget users.

So, when a login is performed by "admin", the search filter will add additional filter and become "(&(&(objectClass=person)(|(cn=admin)(cn=cat)(cn=jack)(cn=john)(cn=jackie)))(cn=admin))". 

You will notice an extra (cn=admin) is added to the search filter to make sure it return only the "admin" user.

User license determines on how many eventual users (sorted alphabetically) from your LDAP/AD can log in into the system. You can make use of this attribute to control amount of users returned from your LDAP.

For the trial license, there is a 3-user limitation: if you would like to perform tests on several test users, you will need to remove all the other users until your test user can be in the top 3 spots in the user list.
Do refer LDAP User Management