How to wire this relay with a H/L (High/Low) jumper to Raspberry Pi?
If the relay is a 5V relay, 5v between the IN pin and GND should energize the coil and switch the relay. If it's not a 5V relay, 5V won't switch it.  You'll need to apply the correct coil voltage (most relays coils are 5, 12 or 24V).
When the coil is energized, a normally closed relay will open and a normally open relay will close.  I think H and L stand for high and low side, with the intention that the current flowing in the relay flows form the H to the L.  This is probably done because it is a DC relay and if the direction of the current is specified, the manufacturer can design the contacts accordingly.
Place an ohmmeter across the H and L pins and see if the resistance changes when you energize/de-energize the coil.