CUDA is Nvidia's conputation toolkit and language. it opens up to developers ways to accelerate tasks using the GPU. GPUs are usually really good at doing 2 things better than CPUs, Floating point math, and linear algebra (think a gpu to be like thousands of dumb smaller cores vs a couple of large cores with multiple instructions like the CPU.)

CUDAs main strength is that it makes programming a lot easier for devs, at the cost that its implementation is black box and proprietary. when you have devs use such a proprietary language, the it makes it significantly harder to jump ship if your work relies on it to make a profit.

Cuda is Nvidia's GPU programming toolkit. It has become the de facto standard for machine learning and AI work as well as various other workloads in which running the program on the graphics card is faster than doing so on the cpu.

Edit: There is more that it does, I am just giving a simplified explanation.