A = All Partitions
e = Enterprise (Cross Site)
D = Identify servers by distinguished name in messages.
By default this does a pull replication - which is how AD works by default. If you want to do a push replication use the following command:
repadmin /syncall /APeD
P = Push
You want to do a push replication if you make changes on a DC and you want to replicate those changes to all other DC's. For example, you make a change on DC1 and you want all other changes to get that change instantly, run repadmin /syncall /APeD on DC1.
For all repadmin syntax please see: