GPL-3.0 — see LICENSE.
Plain English: use it, change it, share it, even sell it. But if you distribute it (or a fork), you must ship the source under the GPL too — no closed-source forks, and per GPL-3 §5(a) a modified version has to carry a prominent notice saying it was modified and when.
Third-party components and their licences are listed in THIRD-PARTY.md.