Cache miss is a state when the requested data by user is not available in cache. Usually it may cause delays (often not too big) and during this delay the data is searched for in the original server.
So when the initial request comes from the user, the data is first of all searched for in cache. In case it is not present in cache, it is called cache miss. As it has already been stated, any cache miss slows the general process of loading web-assets because the data has to be searched for on the original server and only then given to the end user. After such procedure all missed files are copied to CDN nodes again and will be served from cache from then on. It is necessary to underline that in case of regular requests for files they will remain in cache. Cache misses usually occur when the file is not “popular” and is not accessed often.