Feedback from a product is what lets the user know what is happening as the product performs, how well the product is performing, and affords the user the ability to make choices in the ongoing use of the product.
Well designed feedback systems are a powerful but often overlooked way of making a product efficient and delightful to use, and are a good way of focusing your designer's mind on decisions regarding functionality, ergonomics, and usability.
Sometimes feedback is obvious, e.g. Is your electric drill working? Yes?/No? It's obvious, right?
Sometimes you need extra help to understand what your tool is doing: Wireless headphones: Power on- LED Blue Light/Rising scale of notes, change in function- different tone, vibration. Power off - descending scale, red LED light. Smartphone: "This button has been pressed!" - haptic feedback (phone vibrates/ phone vibrates differently for different functions etc)
The main categories for feedback through a product's interfaces are (*2021) Physical and Electronic, which may be further subdivided in both areas as: Haptic (touch) Visual, and Auditory.
To communicate functionality with the user maximise the use of feedback in your product by exploiting as many human senses as possible.