Means..
n=0, m=0
1. if Buffer[n] = 'null' then stop
2. if Buffer[n] = 'space' then n = n + 1, m = m + 1, back to step 1
3. m = m + 1
4. Buffer[n] = Buffer[m], back to step 1

>>How can I monitor every little step of the function like that?

1) use a debugger

or
2) take a short test string and follow it manually. For example: "H W". Initially variables i and j are both 0, so it will just copy the character 'H' onto itself, then increment j so that j points to the space. Next it asks if i points to a space, and if it does not then incremenet i.

Second loop iteration: i and j are both 1 and both point to the space. So it will copy the space onto itself, then ask if i points to space. It does in this case, so it will NOT increment i.

Third loop iteration: i = 1 and j = 2 so it copies the 'W' onto the space. j is incremented to 3 and since the character at i is now a 'W' instead of a space the value of i is incremented to 2.
The string now looks like this: "HWW"

Fourth loop iteration: i = 2 and j = 3. The string's null terminator is copied to the second 'W' in the string so that the string is contains this: "HW". The whole thing stops because it just copied the '\0' character.

Mr,Ancient Dragon
you said:
>>>>Next it asks if i points to a space, and if it does not then incremenet i.

how can you decide it, why not j?

When I want to thoroughly test something I take it one TINY step at a time, but that might have a lot to do with my OCD

I would worry about the first step, ensure that works fine, then move on to testing the rest of them, rather than trying to monitor everything all at once.

I can break the execution of the following code before the compiler starting to execute it or after execution of it.
But the line contains many steps, I simply want to trace it, How?

Read the code! Does the code say j? NOOOOOO

First do whatever is inside the parentheses, then do the rest.

1) calculate the address Pbuffer+i -- increments pointer Pbuffer by the value of i, and store that value somewhere.

2) calcualte Pbuffer + j and saves that someplace else.

3) increment j

4) copy the character at the pointer calculated in 2) above to the address calculated by 1) above. If i and j had the same value then both 1) and 2) would result in the same address.

5) if the character at the address calculated in 2) above is '\0', then stop the loop.

Don't give me a fish, teach me how to fish.

Just trying to answer your question. If you don't like the answer I gave you, then I'll shut up.