This is the flag the al-Qassam Brigades. It is a stylized version of the Shahada, the Muslim declaration of faith reading: I bear witness that there is no god but God, and I bear witness that Muhammad is the Messenger of God.

Hamas’s stated goal is to destroy the state of Israel and impose Sharia law on the whole the region. *

They are a designated foreign terrorist organization in many counties, including the US, UK, Canada, New Zealand, Australia, the whole of the European Union, Japan, and Israel.

Depending on where you live, it may be illegal to fly the flag of a terrorist organization.

*from the Hamas Charter:

The Islamic Resistance Movement believes that the land of Palestine is an Islamic Waqf consecrated for future Moslem generations until Judgement Day. It, or any part of it, should not be squandered: it, or any part of it, should not be given up. Neither a single Arab country nor all Arab countries, neither any king or president, nor all the kings and presidents, neither any organization nor all of them, be they Palestinian or Arab, possess the right to do that. Palestine is an Islamic Waqf land consecrated for Moslem generations until Judgement Day. This being so, who could claim to have the right to represent Moslem generations till Judgement Day