The copious amount of side-notes [1][2][3] is likely to clarify most doubts, if one is willing to go through it. Obviously "contract law applied to software" is not a fun read though.

[1] https://www.gnu.org/licenses/gpl-faq.html

[2] https://www.gnu.org/licenses/gpl-howto.html

[3] https://www.gnu.org/licenses/quick-guide-gplv3.html