What is "Set the merge agent on the problem subscribers to use the slow link agent profile"

I have never used Merge Replication Feature of the sql server before, so What does "Set the merge agent on the problem subscribers to use the slow link agent profile" mean?

Background info:

"Merge replication has been configured for an application that is distributed across offices throughout a WAN, many of the tables involved in replication use the xml and varchar (max) data types. Occasionally, merge replication fails due to timeout errors. You need to reduce the occurrence of these timeout errors. What should you do "