The idea of vector space is that it is closed under vector addition and scaler multiplication i.e linear combination
Formally ,
Technically, V is a real vector space. All of the theory also holds for a complex vector space in which the scalars are complex numbers.
The main idea is that subspace is a space inside $R^n$ which is also a vector space i.e satisfies the condition of closedness under linear combination
Formally,
In determinant , we saw that when determinant is 0 a lot of vector gets mapped to 0 vector via the linear transformation. The set of all these vectors compound to null space .