What exactly is an EStoppel??

[Angela Henderson]

Can someone explain what an estoppel is and when and how does it get used? I've read about it in a few responses.  It seems as if it's a legal document that helps to validate information about existing tenants?  When is it appropriate to use this document?  We're looking into a triplex as our second investment property and most seem to have tenants. Is this document assurance of what tenants are paying and lease duration?  What happens to existing security deposits that tenants have paid to the previous owner?

Any help is appreciated! thanks!