So, when you zip a file in most OSes (windows, macOS, Linux, etc.) It compresses the data to be easier to move around
Zip-bombs are VERY large files that have been zipped to a much smaller size, and when you unzip them(which is mandatory to access the data once zipped) the computer has to devote all resources to the file(s) making the computer basically unusable untill the entire file is unzipped