They are very versatile, so it depends on what you want to do with them. If you know how they work, it should only take a bit of thought to figure out.
strStr takes two arguments, and searches through the first argument, trying to find a copy of the second one. If it does, it returns the position (how many characters after the start, so it would be 0 if you used strStr("food!", "foo"), or 5 if you used strStr("blargablar", "abl")), or -1 if it can't find it.
Since it returns a number, you can use it anywhere you could use a number. In general, however, you would use it to see if a string contains a specific character or phrase (checking if it is <= 0 or < -1), or anywhere else that you care where/if a string contains a second string.
strLen takes a single argument, and returns how many characters it has.
Both functions do nothing on their own, but they return useful numbers. In general, you would store the result in a variable (%len = strLen("someText"), %pos = strStr(%param, "I"), or %len = strLen(%obj.name) would be valid examples), or use them directly in equations (if(strStr(%message, "kill") != -1), strLen(%var) + 42)
It really depends on what you want to do, and it's all logic. Keep in mind that coding is not voodoo, you don't just wave your hands and type some meaningless cryptic phrase and everything works, there is a logical reason for everything. Unfortunately, some people ask for "help" and really just want the mystical incantation that makes magical code happen. If you put some thought into it, you can outdo them all...
Edit: If you are doing simpler things, like storing data in a string, look into the word, field, and record functions. They let you work with a string as if it was split by spaces, tabs, or newlines.
It's rather hard to give the best advise possible when you don't know if someone knows about any specific functions or not, or what their skill level is...
