Hiya Jen,
Basically "Estancia" means you have permission to stay in Spain ie you have the visa and is normal on Spanish schengen visas.
With regards to the length of stay the Spanish are a little bit tight on the duration for their schengen visas especially if it's the first time you have applied unlike the French or Portuguese who normally give 6 months on first time applications.
Your validity of 15 days maximum stay is correct for a visa valid for 30 days under the schengen rules, anyway glad you got it and have a good time...
