2 Answers

To guarantee against any loss which another might suffer. Example: two parties settle a dispute over a contract, and one of them may agree to pay any claims which may arise from the contract, holding the other harmless.

"Indemnify the land owner against any third party claims" basically means that the land owner may lease the land to say, Walmart, for 30 years.  Under the terms of the lease, the fee owner may require Walmart to take out (and show proof of) additional insurance indemnifying the land owner against the liability to pay damages for death, bodily injury or damage to the property.

......unless, usually there's an "unless", it it through the GROSS negligence of the land owner.  For example, someone got hurt driving car into a deep sink hole underneath the property that the land owner knew about but never disclosed to the building owner or Walmart.  Such gross negligence could invalidate any indemnification called for and agreed to in the master lease.