The copyright gives the control over a work in the hands of the copyright holder. This can be the author of a work, or someone else if the author has transfered the copyright to another legal person.
Usually, a copyright is used to prohibit copying of a work. In the free software case, we explicitely allow unlimited use, distribution and copying of our work. Often we use our copyright to enforce the freeness of the software, and call this copyleft. So, if you make modifications to the program, and distribute the new program to someone else, you have to do so under the terms of the same copyright. This is the case with the GNU GPL, but not with the BSD style licenses.
Turtle is licensed to anyone under the terms of the GNU GPL. There is more information about free software and this license on the GNU web site. Please take your time to read and understand the terms of this license, as it is integral to our success that you follow it, and know how it works. The license is not intended to limit your freedom, but to protect it for the future.