The Euríbor (Euro Interbank Offered Rate) is the average rate at which eurozone banks lend money to one another over various terms. In Spain, the 12-month rate is the one most commonly used as the reference index for variable-rate mortgages.
How it affects your mortgage
On a variable-rate mortgage, the payment is recalculated periodically (usually every 6 or 12 months) by adding the agreed spread to the Euríbor value at that time. If the Euríbor goes up, your payment goes up; if it goes down, your payment goes down.
Relationship with the ECB
The Euríbor largely tracks the interest-rate decisions of the European Central Bank, although they aren't exactly the same thing: the ECB sets the official reference rate, while the Euríbor reflects the interbank market's expectations about those rates.