Herot (or Heorot, in some versions) is the name Hrothgar's mead hall in the epic poem Beowulf. It is the main fortress for the Danes and a major symbol of wealth, power, and control. Before Grendel begins attacking the fortress, one can view it as a symbol of security, order, and civilization, as a structured haven safe from the chaos of the wilderness.
After Grendel attacks Herot, the hall is understandably left in disarray, and no Dane is brave enough to spend too much time there, especially after dark. Essentially, Grendel's raids subvert Herot's image as a safe fortress, and so the order of Hrothgar's reign is accordingly also brought into question. In a nutshell, after Grendel's attacks Herot comes to symbolize disorder, chaos, and weakness.
It is impossible not to read a religious layer onto this idea. Grendel is generally taken to be a descendent of Cain, the exiled Biblical character who murdered his brother, Abel. As such, Grendel represents the heathen barbarity cast out of God's community. In contrast, Hrothgar's Herot can be seen as a symbol of Christian order and security, as a community favored by God. As such, the different phases of Herot can also be seen as different phases of Christian spirituality; on the one hand, you have the peace of God's approval and, on the other, you have the disarray following the dismantling chaos of "heathen" culture. In many ways, therefore, Herot's development can be seen as a commentary on what is perceived to be religious virtue as opposed to what is not.