Hi am.sharan hope all is well!
The callback system in the fastai library allows you to execute any code at specific times in the training. This can be extremely useful if you want to
record some parameters (for instance metrics, gradients) use a condition to change a parameter (for instance change the learning rate if the validation loss hasn't improved) make a schedule of parameters (for instance changing the probabilities of the dropout layers as the training goes)
The above quote is from a notebook in this link Using the callback system in fastai
I also found these posts useful to. Implementing callbacks in fast.ai | by Edward Easling | Medium
What the heck is a callback?!?!?! | by Mark praire | AI³ | Theory, Practice, Business | Medium
Cheers mrfabulous1 