An IRQ (interrupt request) value is an assigned location where the computer can expect a particular device to interrupt it when the device sends the computer signals about its operation. For example, when a printer has finished printing, it sends an interrupt signal to the computer. The signal momentarily interrupts the computer so that it can decide what processing to do next. Since multiple signals to the computer on the same interrupt line might not be understood by the computer, a unique value must be specified for each device and its path to the computer. Prior to Plug-and Play (PnP) devices, users often had to set IRQ values manually (or be aware of them) when adding a new device to a computer.
If you add a device that does not support Pnp, the manufacturer will hopefully provide explicit directions on how to assign IRQ values for it. If you don't know what IRQ value to specify, you'll probably save time by calling the technical support phone number for the device manufacturer and asking.