What Is the Purpose of a Debt Validation Letter?

debt

Can someone lead me in the right direction? I’m very baffled on what my first step is. I have all 3 of my credit reports. I have medical and credit card debt. The medical is with collection agenicies and the credit cards are with them still(no collection agency as of yet). I keep reading about debt validation. Why? And with all of them or just collection agencies?