HOOKD3DAPI

I am having trouble understanding this code. Can someone explain what it means?

This is a define. Basically it replaces a certain string of your code with another. This is a bit hard to understand, so here is an example:
Now, what this does is everytime your compiler sees myOutputDebugString, it will replace that string token(Basically that entire word) in the document and replace it with add_log, but you won't be able to see it, it will be done automatically. Now it seems like it isnt that useful, after all you can just use quick replace. But you see, you can do much more with it.

[code]
#define MAX(a, b) ((a) > (b) ? (a) : (b))
[code]
The code above is a pretty standard macro. It returns the max of two variables. So basically it allows to make complicated tedious code quite neat. If you wish to know more, or a more technical/correct version of what I said check out MSDN: The #define Directive (C/C++)
And a less technical, but more acurate, explained better cppreference here: #define [C++ Reference]

Now lets take a look at this one:
So in your code...
Line 1: The define. Since HookD3DAPI is defined already theres no need to write out all the stuff below it. But I understand that someone has just replaced the a and b in the original function with the names of the current functions to make it easier to read. So, if you were making this an ACTUAL function, make sure to eliminate the first line.

Line 2: This is the check. Get the current address pointer for Endscene (42 is the offset for endscene) and compare it with our own hooked function. If its not the same...

Line 3: Set our return to the original endscene. This is why we call pEndscene(params) at the end of our hooked function.

Line 4: Write over endscene's address with our own. This successfully allows us to add our own instructions in because when Endscene is called, instead of using the original Endscene, the new address links it directly to our own function.

Line 5: Since HS has a copy of the original Endscene address, we must also change HS's version to match our new hooked function. That is why we retrieve the Endscene api from HS and write over it with our own again, to prevent detection.

Now I realize this is a lot to process. You might not know C++. All I ask is that you don't start out running. Learn basic syntax then mature gradually into this. You learn more that way and you learn faster that way. Best of luck to you.