PyInstaller is distributed under the GPL license (see the file COPYING.txt in the source code), with a special exception which allows to use PyInstaller to build and distribute non-free programs (including commercial ones). In other words, you have no restrictions in using PyInstaller as-is, but any kind of modifications to it will have to comply with the GPL license. See also our FAQ.
Help keeping PyInstaller alive: Maintaining PyInstaller is a huge amount of work. PyInstaller development can only continue if users and companies provide sustainable funding. See Funding PyInstaller for how to support PyInstaller.