This duplexer is VHF 150-159 MHz, (9 MHz minimum separation).
Duplexers allow bi-directional (transmit TX and receive RX) communication over a single path (i.e. antenna) at the same time. The duplexer isolates the receiver from the transmitter while permitting them to share a common antenna. In other words, the duplexer prevents the receiver and transmitter from "hearing" each other.