CS fallback stands for Circuit Switching fall back. As we know LTE is a packet based technology, to utilize circuit switching based services LTE network fall back to circuit switching domain.
The CS fallback in EPS enables the provisioning of voice and other CS-domain services (e.g. CS UDI video/ LCS/ USSD) by reuse of CS infrastructure when the UE is served by E‑UTRAN. A CS fallback enabled terminal, connected to E‑UTRAN may use GERAN or UTRAN to connect to the CS‑domain. This function is only available in case E‑UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.