Euribor (12 month) is the interest rate most commonly used to calculate mortgage payments in Spain. It is used for both variable and fixed rate mortgages in Spain.
Fundamentally, Euribor is a Euro system interbank lending rate determined by the key interest rate (on main refinancing operations) set by the European Central Bank (ECB).
Basically, in terms that most people would understand, Euribor (12 month) is the interest rate most commonly used to calculate mortgage payments in Spain. It is the interest rate that banks in the Euro Zone use to lend to each other, and is derived from the base rate set by the ECB.
So when the base rate goes up, so does Euribor, which in turn pushes up the variable mortgage interest rates in the Euro-zone. Most Spanish mortgages with variable rates are calculated as Euribor + X%, where X is normally anything between 0.75% and 2%.