In the explanations below, the prefix "0x" signifies that the number is expressed in hexadecimal digits.  Instance ID refers to the number that shows in the Instance column of s3pe - part of the unique name of any packed resource.

A string is a sentence or phrase or even just one word or letter written in normal printable characters.  It is usually intended for humans to read.  In Sims 3 context, you will most commonly see strings used for the name or description of an object in the catalog, or the words that appear for the interactions in an object's menu.

STBL (String table) is a resource storing the human readable words you see in the game.  An individual STBL resource holds one or more strings in only one language at a time.  The game understands which language it is by the first two hex digits of the Instance ID.  Example: 0x0089635529748256 contains English strings, while 0x0489635529748256 will hold the strings for Danish translation.   Any STBL beginning 00 is for English-speaking countries, and any beginning 04 will be used by the game to display words to people in Denmark.

String set.  This is a set of STBLs that all end with the same 14 hex digits.  Each one will have a different first two digits of course, to show what locale that STBL is for.  By convention, each STBL in the set will have the same strings in the same order (just in a different language) but technically it is not a requirement - strings can actually be in any order and in any STBL instance number at random *as long as the first two Instance digits are correct for the locale* - see String GUID

String GUID.  At the end of the day, the way a game decides what string to show is by its GUID.   The GUID is the same across each language. For example:

English:          STBL InstanceID=0x0089635529748256, GUID=0x1234234534564567, String="My aunt's pen"

French:            STBL InstanceID=0x0789635529748256, GUID=0x1234234534564567, String="La plume de ma tante"

But the STBL InstanceID for the French could have been 0x07EDCBA987654321 and provided the GUID was still 0x1234234534564567 and the first two digits of the InstanceID=07 (France), the game could have found it.  However, it makes life easier to stick to convention of using the same STBL numbering set for the same GUID strings.

The GUID is not a random number.  It has to match the FNV64 hash of a seed string defined in the C# script.  If you are a script modder you will almost certainly already know how this is done.  If not, you need to be aware that you must not change the GUID of any string, otherwise the game will not be able to use it.  s3oc and TSR Workshop change name and description string guids to make them unique when cloning an object, but both tools know what they have to hash to make it work.