When an individual has access to your information this could be potentially dangerous. The most common way impostering happens is when someone uses your personal information to sign up for things online such as loans or credit cards.
Another way of impostering is someone simply acting as if they are you while making a purchase or doing a transaction. For instance using a card with your name and having an I.D with your name on it and they really are not you, or going into a bank and withdrawing or transferring funds.
Here's a few ways you can prevent this from happening to you.
Frequently manage bank statements online and make sure purchases look familiar
Check your credit score often to verify everything shown is correct
Update passwords more often than normal
When setting up passwords or security questions, make the answers something only yourself would know