The Transport network in telecommunication systems consists mainly of two parts:

the Backhaul and Fronthaul, where the backhaul connects the Radio access network RAN with the core network. 5G backhaul is essentially similar to 4G backhaul but carrying much more traffic due to higher performance and higher bandwidth 5G New Radios

Before defining the Fronthaul, some terms should be explained:

The RAN consists mainly of the Base Band Unit (BBU) and the Remote Radio Unit (RRU)

In 5G networks, the BBU is disaggregated into a Central Unit (CU) and a Distributed Unit (DU). And the RRU is represented by the Radio Unit (RU)

In other words the RAN system in 5G networks consists of the the Central Unit (CU), Distributed Unit (DU) and Radio unit (RU)

The Fronthaul defined as the fiber-based connection in RAN infrastructure between the Base Band Unit (BBU) and Remote Radio Unit (RRU). More specifically, the fronthaul interconnects the DU to the RU. The 5G midhaul is a new concept introduced in 5G which interconnects the CU to the DU. 

The RAN architecture can be designed represented in different architecture designs as represented in the following figure: