The due diligence period is the contract window in which a buyer inspects the property, lease, title, and financials before deposits go hard.
This is the buyer's opportunity to verify what the offering memorandum says. Typical items include the lease and amendments, estoppel certificates, tenant financials, title and survey, environmental reports, the property inspection, and zoning. If something does not check out, the buyer can usually renegotiate or terminate and recover the deposit, depending on the contract. The length of the period is negotiated, so it helps to line up third-party reports as soon as the contract is signed.