As others have mentioned, Bitcoin is a wallet software and a payment protocol. What you use it for (the moving around of accounting units) is nothing but a result of how you run the software, but the core software is 100% freely available and the protocol by which all Bitcoin participants must communicate is 100% transparent. That is what open source means in this context.

Contrast, please:

Interested but uninformed Linux user A: "How do I make XYZ change to the kernel?"

Linuxhead B: "Well, you log into www.linuxheadconsulting.com and after agreeing to some fees..."

A: But I thought Linux was open source?

B: Fine then, spend 20 years learning the arcane code and put in all the work to do it yourself. See if I care.

A: But it's haaaaaaard..

----

Interested but uninformed person A: "how do I get them?"

Bithead B: "well you log onto www.mtgox.com and after agreeing to some fees..."

A: But I thought Bitcoin was open source?

B: Fine then, build or purchase a mining rig and compete against the rest of the world, or post a buy ad to localbitcoins.com and do it yourself. See if I care.

A: But it's haaaaaard..