This is a certificate evidencing that you do not have a criminal record in Spain.
This website:
tells you all you need to know to go about obtaining one but it is all in Spanish. The cost of this certificate is 3.5€. You can download the application form from the website.
If you cannot find someone fluent in Spanish to help you to identify the requirements and fill out the forms, etc., then I suggest you go to a Gestoria near you and ask them to do it for you. Of course, they provide this service for a fee but it should not be too steep.
Also please note that if you are going to use this certificate outside Spain, it has to be legalized with the Apostille. Make sure you indicate this requirement when you apply for the form. The Apostille has an additional cost but it should not be very high.
P.D. When I tried to post this reply I got an Error message telling me web links cannot be included, but you can find the website by googling: certificado de buena conducta España. It is a Spanish Ministry of Justice offical site. Sorry about that.