This is 4-Channel Solid-State Relay Module, it is Arduino compatible. Solid-State Relay SSR is some what similar to that of a mechanical relay & the difference is that, there is no moving contact in a Solid-State Relay. Here we make use of Phototriac coupler to turn it ON and OFF. It finds its application in AC circuits as a TRIAC has got only limited applications in DC. And with that, this relay is suitable to control AC load, not for DC load.
Solid state relays do have several advantages over mechanical relays. One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there’s no moving contacts, solid state relays can be switched much faster and for much longer periods without wearing out.
Features:
4-Channel Solid-State Relay
Rated output load: 100V ~ 240V AC (50Hz/ 60Hz) 2A per channel
Rated input power: 5V, 160mA (all channels ON)
Triggered at Low level (or Active Low)
Separate LEDs showing status of each Solid-State Relay
Input control signal:
Low Signal (Solid-State Relay ON / CLOSE), 0V- 2V or GND
High Signal (Solid-State Relay OFF / OPEN), 3.3V - 5.0V or leave it floating
Dimension: 57mm x 55mm x 25mm
Offer two types of input control terminals: Header pin and Terminal Block
Module interface:  (Input)
DC+: connect the power supply positive (relay voltage power supply), minimum 5V, maximum 12V.
DC-: the cathode of a power supply 
CH1: 1 relay module signal trigger end (low level trigger effective) 
CH2: 2 relay module signal trigger end (low level trigger effective) 
CH3: 3 relay module signal trigger end (low level trigger effective) 
CH4: 4 relay module signal trigger end (low level trigger effective)
 
 
Product Highlights
Omron 5V solid state relays (240V, 2A), output port with resistance-type fuse (240V, 2A)
Input power source: 5V DC (160MA)
Input control signal voltage: 0- 2.5V state low level and the relay is OFF, 3.3-5V state high level and the relay is ON.
With 2.54 CM pin header and blue KF301 terminal, more convenient for control line.