How does this do what it does (C++ classes, use of colon)

[PieterP]

@killzone_kid

It’s a wrapper over c array and it is not available on Arduino

That's why the “if you can” is there, it is available on all Arduinos except AVR.
And even if you're stuck on AVR, you can implement it yourself in under 10 lines of code.

@killzone_kid

Besides, just because one can doesn’t mean one should.

In this case, you use it because it solves the problem of initializing an array member variable, which is not possible using C-style arrays.