Open Source refers to a whole group of licensing schemes for software. The common factor is that the user of the software has access to the source code for the software (without having to sign things like non-disclosure agreements) and they will typically (though I'm not sure if this is universal) also have the ability to produce their own builds of the software from the source and produce patches and updates for the source.
There are lots of open source software licenses. Tcl uses one of the common types, the BSD license. Others you might well have heard of are the GPL and the LGPL, both designed by the FSF. There are many others though. [Extend and explain with a link list...]