Brigades

So, when the news says there are a number of Hamas "battalions" in the city of Rafah, what does that mean?

Wikipedia actually has a page on the Izz ad-Din al-Qassam Brigades.

The Izz ad-Din al-Qassam Brigades (abbreviated as IQB); ...  'Battalions of martyr Izz ad-Din al-Qassam'; often shortened to Al-Qassam Brigades, named after Izz ad-Din al-Qassam, is the military wing of the Palestinian organization Hamas. Currently led by Mohammed Deif and his deputy, Marwan Issa, IQB is the largest and best-equipped militant group operating within Gaza today ....

The forces are mainly divided into five brigades, divided geographically. Each brigade is divided into multiple battalions, with 30 total battalions. Each battalion is associated with a major settlement ....

During the 2023 Gaza war, the IDF published its intelligence about the Hamas military in the Gaza Strip. They put the strength of the Qassam Brigades there at the start of the war at 30,000 fighters, organised by area in five brigades, consisting in total of 24 battalions and c. 140 companies. Each regional brigade had a number of strongholds and outposts, and included specialised arrays for rocket firing, anti-tank missiles, air defenses, snipers, and engineering ....

As always, to see more and to see the footnotes and sources, go to the Wikipedia page.