Well it's clear that you need to get an official document authenticated in one country (presumably Spain) so that it can be recognised by the authorities in another country (the UK?).
Here in the UK, a 'notary public' can do the task (as well as a solicitor) but many people would go straight to a solicitor anyway (largely because it's easier to find a solicitor than a notary public). In Spain you've got 'notarios' as well as 'procuradors'.
However what isn't clear from your post is exactly which document it is that you need to get authenticated! If re-reading the letter (which you've presumably received) doesn't help, I can only suggest asking whoever wrote it to tell you exactly what it is that they need.
This link explains what 'Apostilles' are:
http://www.sloughnotary.co.uk/hauge-convention.php