What does "downloadable" mean and how is it achieved by the general public user?

Perhaps your question is very simple, or I'm not getting it.
Downloadable is a file that resides on a remote machine (usually a
computer server) and is available for someone to get (copy) it to
his/her machine (pc, mobile phone etc).
How its achieved?
- If you mean how a public user can download the file to his machine,
this depends on the protocol used by the remote machine in order to
make the file downloadable. The most common case is that the file
resides on a an http web server, so the end-user simply must open
his/her web browser and type the address of the file (eg.
http://www.servername.com/filename.zip)
- If you mean how a public user can make a file of his own to be
downloadable for other people, he/she must do what the above server
does. This can be done with many ways. Most people just find free or
buy web space to do it (eg. you can find free space in
www.geocities.com). This space actually works like a remote hard disk
which you can access and copy files there, which automatically are
available for others to get (downloadables).

With the advent of streaming data feeds for sound and video,
"downloadable" can also mean that you have the option to
download the whole file to your PC (and then later hear/view/use it off-line),
as opposed to only viewing it on-line via a streaming data connection.

Much streaming content fails over slow modems (mine is a 56K modem).
Some streaming content (like Yahoo! Launch music videos)
  is provided at VERY low resolution if you're on a slow modem.
So sometimes the only way to enjoy the content is to download the whole
file to your PC, then open it with RealPlayer or QuickTime or Windows
Media Viewer or Adobe Acrobat Reader or whatever.

But much content is protected from users copying and re-distributing it
by only making it available in streaming format; also, the whole time you're
enjoying streaming content, the web site can be throwing ads at you.

It mainly means that you can move that complete data set to your
computer from the remote computer/server. Most cases server. IE:
Website to your machine.