It doesn't apply just to apartments. As Nova says, it's the minimum rate of interest the mortgage will attract, written into the mortgage, regardless of how low the Euribor actually goes. Thus someone could take out a mortgage when the interest rate is at 6%, and the mortgage escritura might say that the "suelo" (i.e. minimum interest rate) is 4%. It cannot go below 4% even if the Euribor goes down to 0.5%.
These "suelo" clauses are in almost every mortgage issued in Spain, and around a week ago, Spain's Supreme Court issued a ruling that the clauses were "abusive" unless "comprehensible and transparent". That definition puts a higher burden of proof on the banks than just "having been made explicit". The Supreme Court actually specified that the banks have to do more than "just be clear", and that banks must demonstrate efforts to ensure mortgage customers understand not just the issue, but its significance and potential consequences.
The Supreme Court said in its ruling that any such clauses from now on will be declared null and void. It's not a retroactive judgment, but clearly in the case of an eviction, there is now legislation underway in any case enabling the person being evicted to appeal to the Court to stop the eviction because of abusive mortgage clauses. Some banks, e.g. BBVA, have announced over the last couple of days that they're investigating abolishing them entirely ... which suggests that all their bloody clausulas suelo are abusive ...
This is my understanding of the situation, anyway.
