Python 3 is the latest major version of Python, a scripting language used my very many open source developers. Python is especially important to Ubuntu; tools like the Software Center, Update Manager, Apport, Ubiquity (Ubuntu's installer), and many more are written in Python.
Many of the Python programs on an Ubuntu system are actually written in Python 2, as there are substantial differences between the two languages and it is not always worthwhile to translate a program from Python 2 to 3. However, new applications and utilities are being written in Python 3, so it is becoming more and more prominent. Eventually Python 2 will be largely phased out.
It is a release goal for Ubuntu 14.04 LTS to have only Python 3 on the desktop CD images.
For developers, it's probably a good idea to learn Python 3 (and use it for any new programs), though developers who want to work on existing Python 2 projects will need to know Python 2.