Network slicing is a term used in 5G network. Since there are different kinds of devices like LTE UEs, IoT devices, Mission-Critical communication devices, creating a separate network for each type of devices is not advisable. Network slicing is a concept which uses virtualization, SDN and NFV concepts to create virtual separated network for each different kind of devices. It reduces the CAPEX/OPEX for the network operator as well as reduces time to the market for new services.
Reference:
https://www.ericsson.com/news/150126-5g-systems-enabling-industry-and-society-transformation