a device that alters (hacks) game mechanics
Thaaaat's a little excessive. It does not alter, let alone hack (unless you're using it in the generic sense that's way too overused these days like 'life hacks') the game in any way. It'd be much more accurate to say it exploits game mechanics.
A pretty basic definition of a macro is it's a script - a short, simple set of instructions to help with automation. For instance, with a keyboard and the necessary software or built-in features you can have a macro where, when you press one key it fires off a sequence of other key presses, and can loop or repeat them. Like keyboard shortcuts on steroids. Since this is done by electronics/computer/whatever, it can do those inputs much faster than an human could hit said sequence. I seem to recall one for BF where you had your sniper out and hit fire, and that one button press would fire the sniper, switch to secondary/pistol, and fire again (maybe repeatedly).
It can also be done with hardware, such as on a controller where a switch can be flipped to change the mode of another switch between a continuous signal or a broken signal at whatever frequency when you hold that second switch down.
Their use in gaming is a huge gray area. Clearly in some cases it can be a tremendous advantage against someone not using the same thing, as some of these rapid fire examples show. But other uses... think of games like World of Warcraft where you can set up complicated sequences of spell casts or timings and other such that go off with a key press instead of having to input it all one step at a time yourself - an now think about how nice that would be when one of your standard attacks you do over and over again has six steps that are always the same.
Most modern gaming hardware (keyboards, mice, controllers) can have some level of this, and the higher-end stuff can have a lot of it - you don't need 11 thumb buttons on a mouse for most applications. Which is another aspect to consider, does a eight button mouse user have an unfair advantage over someone who only has four?
But there are some cases, like the rapid fire or this walk fast thing, where macros can be used to exploit... well if not poor game coding at least not forward thinking. In the past you didn't usually put in a limit to inputs where you're looking at human interaction, because a human's reaction time and ability to repeat something like a mouse click does have something of an upper limit. But with the advent of all the 'advanced' controllers and use of macros, you're no longer limited by basic human physiology, so things like this burst fire weapon RoF cap have to be added. And in the case of the walk thing, that's just broken period - you don't need a macro to do it, but a macro will definitely make it easier to consistently repeat it to maximum effect, so it's exploiting an issue already there.
This is mostly for /u/Red765/ benefit or anyone else with a similar knowledge of the issue. I'm not here to get into a debate about when and where macros are 'cheating'.