What is Transport Layers and Transport Routes in SAP ( TMS )

All development projects developed in the same SAP System and transported on the same transport routes are grouped together to form a transport layer. 
Before you start the first development project, you create a transport layer in the TMS transport route editor. This transport layer is assigned to the development system as its standard transport layer. Objects delivered by SAP belong to the transport layer "SAP". Other transport layers are generally only needed when new development systems are included in the system group.
After you have set up the transport layer you set up the transport routes. There are two types of transport routes. First, you set up consolidation routes, and then you set up delivery routes:

1. Consolidation routes
To make your changes transportable, set up a consolidation route for each transport layer. Specify your development system as the starting point (source) of these consolidation routes. Specify the quality assurance system as the transport target (in a two-system landscape, specify the production system as the transport target).
2. Delivery routes

After you have imported your development work into the quality assurance system, you then want to transport it into your production system.( Quality as source & Production as target) You may even want to transport it into several SAP Systems (for example, additional training systems). To do this, you have to set up delivery routes.
Delivery routes have a source system and a target system.
When you set up a delivery route, you are making sure that all change requests that are imported into the route’s source system are automatically flagged for import into the route’s target system.
You can set up several delivery routes with the same source system and different target systems (parallel forwarding). You can also set up delivery routes in sequence (multilevel forwarding).CTS transport control makes sure that all requests from the development system are flagged for import into all other SAP Systems in the same order in which they were exported. This is important, since different requests can contain the same Repository object or the same Customizing setting at different development levels, and you must avoid overwriting a more recent version with an older version.