What does the common terminal (COM) do in PLC
COM is the common connection for relay or transistor outputs; essentially when the output is activated, the PLC will energize the relay or transistor, effectively connecting the NO (normally open) pin to the COM (common) pin.

Some PLCs have form-C contacts, which would have NO, COM and NC (normally closed) contacts. When the output is off, the COM and NC would be connected, and again, when the output is on, the COM and NO contacts would be connected.

If you wanted to activate a light (or buzzer, or actuator, or... or...) you would most likely connect the COM to the power source, one leg of the device you want to control to the NO contact, and the other leg of the device to the supply return. (there are other wiring options, but I suggest this one as it's often the safest, although it will not work for most transistor outputs). It might be easier now to visualize how the NO and COM contacts form a switch that closes when the output is turned on.

You don't specify the particular I/O board you have in this PLC, so a word of caution should be said. It is not uncommon for an I/O board to save space by having only one COM and a dozen NO contacts. Any of the outputs that are set would connect that output's NO contact to the COM rail (it's now called a rail since it is very much like the neutral bar in your breaker panel in your house). The outputs will not be isolated from each other if this is the case, so you need to be careful and plan your I/O accordingly. For relay outputs it is usually the case that each output will have its own COM contact, but it is very common to have a single COM rail for transistor outputs.

It is also important to note what the outputs are rated for; transistor outputs are not designed to drive much other than low current indicators or other digital inputs, and relay outputs might be able to handle an Ampere or so for a resistive load or half as much for inductive loads. The output module should list its ratings somewhere.

Finally, a note to help prevent frustration with transistor outputs. Many digital outputs that employ transistors will use a single NPN transistor per output. The emitter will connect to the COM terminal and the collector to the NO terminal. This would mean that the COM terminal should be connected to the common or return or - side of the power supply (not the chassis ground) and that the device to control would be between the positive supply and the NO terminal. When the output is activated the transistor is switched on and essentially connects the NO terminal to COM, completing the circuit and activating whatever it is you have connected. These type of transistor outputs are called "open collector" outputs and cannot send a "high" signal. Many, many people get frustrated because they can't supply a positive voltage with this kind of output.