When we percieve or interpret a communication, event or object in the way it was intended, we are said to have understood.

The definition of understanding is fairly straight forward.  Philosophically the concept is a little more complex.  For example, what makes your percieved understanding correct, assuming it is different from mine?

Absolute Meaning

Does every communication or observed object have a singular and absolute meaning? Ergo, is their a correct intepretation or perception and incorrect one?

One way to answer this question is from the perspective of consequences.  Our interpretation may result in unexpected consequences from those we had logically and intuitively derived from our understanding.

For example, say a shop owner tells you to go ahead and take some merchandise.  You do so without paying and are arrested for stealing as you leave the store.

Despite your protests at this unexpected, and from your view incorrect, consequence, it is clarified you misunderstood.  

The merchant intended for you to understand the merchandise was yours for the taking if you compensated him (paid for it).  

Depending on the details of the communication arguments could be made for both perspectives and each parties understanding possibly has some validity.

Validation

While it is not always effective, the above exaggerated example highlights the value of validating your understanding.  

This can be through questions or actions but can avoid cases where misunderstandings lead to unexpected and perhaps unpleasant consequences.