What Exactly is A "Responsive Theme"??

[Big Game]

I'm not exactly sure what "responsive" means in this situation.

What has lead me to this question is that Google has informed me that my site is not "mobile friendly".
Therefore, in the process of trying to fix this particular problem, I have noticed the term "responsive".
I am inclined to believe that this is when the theme will respond to the browser and act accordingly. Is this essentially correct?
If this is the case, this would mean that if I log in from a PC that my default theme will act. But if I log in to the same page with a mobile device, the responsive theme (or mod?) will automatically recognize that I am using a mobile device and show the mobile theme?
If this is so, can someone please point me in the direction of making this happen?
I have uploaded the responsive curve mod to my site but see no changes.
Also, the default smf theme is mobile friendly but not "responsive"?
Thanks in advance for anyone who takes their time to help me out with this.

[Bloc]

Being responsive means the theme show a optimized version for both mobile/desktop, but with the same code. No separate theme.

Search for themes that are "responsive", there are a few around.

[Glyph]

if you look around at bootstrap designed websites almost all of them are "responsive"

http://www.tutorialrepublic.com/twitter-bootstrap-tutorial/bootstrap-responsive-layout.php
^ Gives up a little bit more information on the question.

[Kindred]

the default smf theme for 2.0.x is Curve - it is not mobile friendly, and not responsive.
The responsive curve mod makes curve (mostly) responsive - to a point...

you won't notice any difference on a PC or wide screen device...
as the screen resolution/size drops, the theme compresses nicely, dropping some information and replacing the menu words with icons.

[Big Game]

Thank You everybody for the response and advice. I checked out responsive themes and looking into redsy.
Also, the tutorial on bootstrap seems a bit overwhelming at first glance but it gave me a better idead of what's going on here and I'm interested at exploring it more.

@Kindred
Thanks again for the advice and help.
So are you saying that all I need to do is install the responsive curve mod and it will automatically make the 2.0x Curve theme responsive ... "to a point"?

[Kindred]

yup...   enough of a point to pass Google's checks... and to look at least decent for your mobile users

see http://test.turtleshellprod.com/index.php

[Big Game]

Wow. Great. I did it and it works fine.
Thanks again!

[Biology Forums]

Press crtl and plus + on your keyboard several times. If the theme converts to fit on your screen, it's responsive (250 to 300 percent is a safe bet on most screens - Chrome)

[Kindred]

Or just reduce the window size to a narrow space ...