What does samplesPerBlock mean in AudioProcessor::prepareToPlay?

On juce audio processor there is:

void prepareToPlay (double sampleRate, int samplesPerBlock) override

what is samplesPerBlock? sampleRate is the ammount of samples per second, for example 48000 samples per second.

The samplesPerBlock tells what is the maximum buffer size the host application is going to use in the subsequent processBlock calls, so you can pre-allocate your possibly needed additional processing buffers/objects.