What does rasterization do to an image?

Without knowing what kind of image you are rasterising, this is kind of a shot in the dark. Yet, here goes:

Rasterisation does exactly what the name suggests: making an image into a raster image, also known as a pixel image or bitmap. Rasterisation is usually done to vector graphics or images that have vector components. Vector components can be things like text objects that haven’t yet been rasterised: the letterforms are vector images.

Examples of vector images are .ai, .eps and .svg files. Examples of images that can contain vector elements are .pdf and .ps. InDesign files (.indd) are also combined raster and vector elements, but .indds are not meant to be distributed to a printer or somesuch: you’d export a .pdf copy of them first. Examples of raster images are .tif and .jpg files.

Rasterisation usually reduces the image to one flat layer, and thus limits editability to a minimum. You will want to keep a non-rasterised version of your file archived at all times, just to make adjustments later, if necessary.