An estoppel certificate is a document used to layout the landlord's and tenant's rights and obligations under an existing lease. Basically, it's the part of your leasing contract that contains things like the date of start and expiration of your lease, the price that the tenant will pay per billing period, the parties' contact information, possible renewal or extension options that the tenant may have, and so on.
The bottom line is the estoppel certificate contains a bunch of important terms that you're going to want to look over before you sign the contract!
Do your due diligence!
Ask for a copy of the lease contract and take it home so that you can look through it and make sure that it's fair. Contracts can often be difficult to understand, so you may even want to hire an attorney to do a quick scan of the documents. Better safe than sorry.
Contracts are legally binding, so it's absolutely imperative that you understand what you're signing. Use the estoppel certificate to your advantage!