Manual:$wgCachePrefix

Details
Overwrite the caching key prefix with custom value. This value is prefixed to the beginning of caching keys to differentiate between caching entries for different wikis stored in the same cache.

If this variable is set to false (default), the cache key defaults to being (Also known as the Wiki id). This is almost always sufficient, and it is unlikely one would want to set this value.

e.g. "cache key: DBNAME:resourceloader:[...]" to "cache key: $wgCachePrefix:resourceloader:[...]"