It's the "insurance policy" that provides the "guarantee" that the rent will be paid for a long term rental (instead of providing another proprerty as collateral).
A garantia is not required for shout term rental..
Technically, you don't need a DNI to "buy" a guarantia and foreign income may be accepted as long as the monthy rent does not exceed 40% of the monthly income.
Apparently, some of the recent Russian arrivals have been able to do this, but many (if not most) are paying higher than "normal" rents without a garantia while they search for properites to buy.
I know an insurance agent you could ask about this, but I can't post their contact information here.