In short it compresses the access-list. The only real impact for you is that you no longer will see the full acl when using the command "show access-list" (not that same as "show run access-list") And will not be able to fetch acl hit counts from Cisco Security Manager
If you have memory issues I guess you also have the firewall in context mode and in a failover setup.
In that case I have a tip for you that will release more memory. (works in ver 9.6(2)3)
- Remove all contexts from the standby firewall (use the console or leave the admin context) - Save the changes on the standby then reload it. - Then push a failover and do the same for the other unit.