The code takes the address of the local variable i to get a pointer into the current stack frame. Then, it aligns the address to 8K page (that is what you do with x & ~8191: 8191 is 2^13 – 1 which means ~8191 is all ones except the low 13 bits, so ANDing it with a number will clear the low 13 bits, i.e. align the number to the nearest lower multiple of 2^13, in other words, align to 8K boundary).
It then takes this address and interprets it as a pointer to a pointer and loads the pointed address from it. See Understanding the getting of task_struct pointer from process kernel stack for further information.
After that, it tries to locate a specific structure stored somewhere after that address: It looks through the following 1024-13 unsigneds, trying to find a place in memory where the current process information (probably) is stored: When it finds a piece of memory holding multiple copies of the current UID and GID, it presumes it has found it. In that case, it modifies it so that the current process gets UID and GID 0, making the process running under root (plus it stores all-ones into the following capability flags).
Cf. struct cred.