The reed relay consists of a switch with magnetic contacts that move under the influence of an external magnetic or the induced field from its solenoid. They have faster switching speed compared to the electromechanical ones but their switching current and voltage is lower mainly because of its contacts thickness.
As you can see above in the first schematic no voltage is applied to the coil so the reed switch is opened and the led is not turned ON. In the second schematic we apply a DC voltage to the coil therefore a magnetic field is produced and the contacts close allowing the current to flow from the battery to the LED turning it ON.
You must know that there is a difference between this type of relay and a reed switch which basically is only a simple switch inside a glass tube that reacts to an external magnetic field and has no solenoid.
Some applications with reed relays