Polling is a method used in software, where a program enters an endless loop to repeatedly check the device status for something new. While this is happening, the processor isn’t doing anything useful, so it cannot perform other useful tasks. This is a problem when there are many devices in a computer to synchronise, nor if exceptional problems occur during regular operation.
This motivates the idea of interrupts, which allows devices to instead alert the processor when something new happens. Why use polling then? Interrupts aren’t really a construct in higher-level languages (like C/C++), so polling is the easiest next thing to do.