LCDS stands for LiveCycle Data Services. If you have worked with BlazeDS then it would be easy to understand LCDS. In Flex several times it is required to do Database Interaction/Operations. At that time either we can use HttpServices, WebServices or RemoteObjects. RemoteObject gives best performance of all three as it communicates with flex client in Binary Format (aka AMF). So BlazeDS & LCDS both enable flex clients to connect with Java RemoteObjects. Beside remote objects both also support Messaging Service (real time data push).
LCDS is just advance BlazeDS having some improvements over BlazeDS. See BlazeDS Vs LCDS
Hope it helps.