Attached is a picture of a 4 channel relay. I have searched the internet for the function of the jumpers circled in red. They are labelled Low Com and High. On all sites which I have looked at they are labelled JD-VCC VCC and GND.
Each channel can be set to be triggered high or low. See User Manual
A high level and low level trigger can be configured as the follows.
Note: This information can be found at end of the document from this post