What does decompose mean in math like decompose the number?

It means take it apart into parts that can be added back together to return to the original number.
For example, you could decompose 37.25 into its integer part (37) and its fractional part (0.25).

Or you could decompose the 2-dimensional vector (2,3) in the Cartesian plane into a horizontal vector of magnitude 2 and a vertical one of magnitude 3.