Let me try to give a more balanced view than the other answer here: I'm English but my mother is Spanish, raised near Barcelona and a couple other cities, and she's fiercely against independence. Let me warn you know, pretty much everything related to this topic is divisive and conested, so get used to 'howevers'.
To begin with, Catalonia/Catalunya is a region in the North-Eastern Corner of Spain, next to France and on the genrerally sunny Mediterranean 'Costa Brava'. With Barcelona and other cities in it, it's probably equal with Madrid as the most prosperous area of Spain, accounting for around a quarter of exports and fifth of GDP with only 16% of the population. Accordingly, it has some of the most developed infrastructure in the country, highlighted with the Port of Barcelona, one of the greatest in the Med. Notably, it very much contributes more to Spain's Government than it gets back. Think of it like the Bay Area or NYC in relation to much of America.
Adding to this distinguished nature, Catalonia is one of the regions have Spain with a clear Pre-Unification history in the Late Medieval era that separatists draw on, although many note this 'Duchy-Kingdom of Catalunya' had different borders and not all that much significance.
Of more fresh, living memory is the legacy of Franco-era Spain, when starting pre-WW2 and into the 1970's the country was run by the fascist Francisco Franco, and he pretty sharply repressed Catalan identity and culture, including a total ban on speaking the similar-but-distinct language, which for the most part became a private affair for Catalan and rebels. This stemmed from the region being one of the most pro-Republican centres in the bloody civil war of 1938 (including one of the few major anarcho-communist), and since Franco's death, there's generally been a strong public resurgence of Catalan identity and pride among citizens.
All this has gone to create some large national divides between Catalonia and the rest of Spain, based both in economic, historical and cultural contrasts.