Castilian Spanish is a type of Spanish that comes from the region of Castile in Spain. This part of Spain used to be really important a long time ago, so their type of Spanish became the most common and influential over time.
Sometimes people call it "European Spanish" or "Peninsular Spanish" too, which means it's the type of Spanish that people speak in Spain.
Castilian Spanish has some differences from other types of Spanish. For example, they pronounce the letter "c" differently if it's before an "e" or an "i". They also use the "vosotros" form when they talk formally to more than one person, which is not used in other Spanish-speaking countries.
Overall, Castilian Spanish is just one way of speaking Spanish and it's important to understand that there are many other varieties of the language. It's a beautiful language that millions of people speak all over the world!