I recommend starting with this tutorial instead:
https://gamebanana.com/tuts/12801You'll be using VS 2017 instead of 2015 (actually, 2019 should also work) and a slightly updated version of the HL SDK which changed some of the C code into C++.
"abs" is a function that returns the absolute value of a number.
For example, abs(-20) would return 20. The ambiguous part means that abs(-20) can be interpreted in multiple ways. In other words, it is confused as to what this "-20" means in my example. It can be a whole number, it can be a real number, yada yada.
As for the overloaded part, don't worry about it for now.
How to fix it depends on the code that is calling that abs() function. However, I advise you to go with the tutorial I posted in the start of my post. (also, while installing VS 2017, check XP support, I believe it is required)