The term "hiding behind civilians" is simply an umbrella term that lays out what is obvious and logical - that Hamas, as a paramilitary, insurgent organization, use their surrounding environment to their advantage by blending in with the public, because they knew previous Israeli ROE prevented them from going after them there. This includes things like what Hamas operatives are themselves describing below at 3:00.
For example, they use civilian ambulances to avoid detection by the IDF. From a Hamas perspective, this would be a logical countermeasure to avoid danger, but to most on the outside, it easily qualifies as using civilian infrastructure to their advantage, because that is by definition, what insurgent groups do. Ambulances can therefore be used to transport the likes of weapons, explosives and so on. At 5:30, the next guy talks about the deliberate storage of weapons and rockets below a school for the very same reasons. The next guy talks about tunnels connecting through hospitals, medical clinics, schools etc.
But its not just the low level guys who get caught that admit it. Senior Hamas members like Mousa Abu Marzouk have also conceded they use tunnels for military purposes. This should surprise no one, because insurgencies, by definition, tend to use all available nearby infrastructure to protect themselves after they launch their attacks and need to protect themselves. What is quite telling about the testimony of the ground level Hamas guys is that they all clearly believe the use of hospitals, ambulances, schools, mosques etc., are great hiding places for weapons, ammo, and rockets, because they genuinely believe the Israelis don't attack those locations.