In your UG level Mathematics you learn some intricate topics which are useful in some advance problem solving such as Vector Calculus, Discrete Mathematics, Real Analysis, Group Theory, Graph Theory, Multivariable Calculus, Topology, Differential Geometry, Abstract Algebra, Number Theory etc. This topics are based on the foundational knowledge of your 11 & 12 th but they are far more intricate and abstract. For example, in Abstract Algebra you will find N-Dimensional Space concept. You will find the concepts of Rings and Fields in group theory etc. This concepts are very abstract and one will need imagination and practice in order to master them. So the courses are difficult and you need to be very sincere about your studies in order to excel the examination.

I hope this answer helps. All the very best for your future endeavors!