Can somebody please translate this peice of code?

Question

Hi,
Please can somebody translate me this peice of code:

So far I have it figured that the input string is read from left to right, and for str, each character is converted to its relevant ASCII value (is this correct?)
From there, str2 is incase no string exists for TextSubjectA
And then str3 has me baffled.
Anybody wish to share with me how this code actually works?
Regards :)