The easy part to explain is "with M/G suffixe" which means that if you set it to 8M, then 8 megabytes would be allocated, or 1G would allocated 1 gigabyte of memory.
The more difficult bit to explain is that it's a cache for storing strings that are used internally by APC when it's compiling and caching opcode.
The config value was introduced in this change and the bulk of the change was to add apc_string.c to the APC project. The main function that is defined in that C file is apc_new_interned_string which is then used in apc_string_pmemcpy in apc_compile.c. the rest of the APC module to store strings.
For example in apc_compile.c
/* private members are stored inside property_info as a mangled
* string of the form:
* \0<classname>\0<membername>\0
*/
CHECK((dst->name = apc_string_pmemcpy((char *)src->name, src->name_length+1, pool TSRMLS_CC)));
When APC goes to store a string, the function apc_new_interned_string looks to see if it that string is already saved in memory by doing a hash on the string, and if it is already stored in memory, it returns the previous instance of the stored string.
Only if that string is not already stored in the cache does a new piece of memory get allocated to store the string.
If you're running PHP with PHP-FPM, I'm 90% confident that the cache of stored strings is shared amongst all the workers in a single pool, but am still double-checking that.
The whole size allocated to storing shared strings is allocated when PHP starts up - it's not allocated dynamically. So it's to be expected that APC shows the 8MB used for the string cache, even though hardly any strings have actually been cached yet.
Edit
Although this answers what it does, I have no idea how to see how much of the shared string buffer is being used, so there's no way of knowing what it should be set to.
