It's complicated unless you read music and/or have trained. I haven't trained, and I don't read music but let's see if I can help.
Don't try and understand chords and octaves and stuff or you will go crazy. You don't need to know all that.
An octave is a group of the 8 notes. And think of the notes on a piano keyboard, as you move along from one side to the other each octave gets higher.
The music keys are G, A, B, C, D, E, F, and back up to G [into the next octave.] And then there's stuff like Chord sequences and stuff. But again, that's too complicated and you don't need to know.
So A is lower than B, B is higher than A, C is higher than B in that octave..... with me so far?
Each key [A, B, C] is called a 'tone' or a 'step.'
Then in between each tone is a semi tone or half step.
And those steps in between are sharps and/or flats.
Sharps are shown as '#' and Flats as 'b'
BUT
and this is where it gets tricky to explain........... G sharp and A flat sound enharmonically the same.
Think like school grades.
B minus is the same as C plus.
A
A-/B+
B
B-/C+
C
C-/D+
geddit?
I work with an Alto Sax player and I sing in Alto [lower female voice] range - and you'd think we would sing and play in the same key, but that's not so.
The sax key will be three semi tones below the copy key I will sing in. So he will be playing stepped down from me - yet we will be in tune.
Here's a list which I have worked out for myself so that when I find a song in a key I can sing comfortably I can then work out what key the sax will have to play it in.
It's not universal because for sax certain notes do not exist, but you'll get the idea.
Higher note at top of list, lower note at bottom, just like school grades.....
Ab/G#
G
Gb/F#
F
[no Fb or E sharp for alto sax]
E
Eb/D#
D
Db/C#
C
[no Cb or B# for alto sax]
B
Bb/A#
A
Ab/G#
G
If G is too high, a semi tone down from that would be in Gb/ F#
and a full tone or step down would be in F.
Two steps down would be in E.
If the copy key for me to sing in was too high, the music would need to be stepped down [transposed] into a key which is comfortable for me.
Sometimes, a semi tone will make all the difference and the listener will not notice too much.
But if it has to be played in a whole key [two semi tones] lower you would start to hear a difference.
And if it's stepped down lower than that you will really know it!
As you know, I sing covers, so the song is already written and it's key is known.
The singer will sing the song in that key. That key is called the 'copy key.'
The alto sax player's key will be three semi tones below the copy key. That's just because an alto sax is naturally tuned to Eb - you don't need to understand why. It just IS!
And that means I'll be singing three semi tones above what the sax is playing in but it will be in tune and sound right.
I hope that helps a bit.
It all gets complicated with guitars because they have strings and can play more than one note at the same time.
A voice can't.
For falsetto, just think Bee Gees and Minnie Ripperton although that's really 'whistle' register and women have that rather than falsetto.
Steve Perry sang high enough without going into falsetto much.
I can't do falsetto OR whistle.
So I can't even attempt to explain any of that.
But I hope I've helped you understand what 'stepping down' means?????
xxx
As I've said before, I just open my mouth and sing.
This might help....................
http://www.josef-k.net/mim/MusicSystem.html
You, of all people, should know I prefer soulful singing as opposed to vocal acrobatics.
Yep. I recall in one thread discussing emoting or something you posted a Mark Anthony live clip too, I think it was 
Now that is