As the name indicates, multi dimensional arrays are arrays that contain more than one dimension, usually two or three but it can have up to 32 dimensions.
A multi array works like a matrix with various levels, take in example a comparison between one, two, and three Dimensions.
One Dimension is your typical array, it looks like a list of elements.
Two Dimensions would look like a Sudoku Grid or an Excel sheet, when initializing the array you would define how many rows and columns the array would have.
Three Dimensions would start to look like Rubik's Cube, when initializing the array you would define rows and columns and layers/depths the array would have.
Further dimensions could be thought as the multiplication of the 3D, so a 4D(1,3,3,3) would be two side-by-side 3D arrays.