In "The Fall of the House of Usher," what is meant by the “fall” of the house of Usher, both literally and symbolically?
You have picked up on a very important aspect of the title. Of course, the "fall" in question works on many levels that are key to the story and the brooding atmosphere of evil that Poe creates through setting and character. There seems to be a real, supernatural kind of relationship between the literal House of Usher as in the location, and its two surviving members, the stricken Madeline and Roderick.
You will want to think about how in the description of the narrator's first sight of the house it is clear that it is given a supernatural menace:
The discolouration of ages had been great. Minute fungi overspread the whole exterior, hanging in a fine tangled web-work from the eaves... In this there was much that reminded me of the specious totality of old woodwork which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air.
Note the important emphasis that is placed on the rot and decay - a rot and decay that is symbolically present in the line of the heirs of the Usher family too, as we discover, for both Roderick and his sister Madeline suffer from a mysterious ailment that has changed them both dramatically. Consider how Roderick is described:
A cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison, lips somewhat thin and very pallid, but of a surpassingly beautiful curve; a nose of delicate Hebrew model, but with a breadth of nostril unusual in similar formations; a finely moulded chin, speaking, in its want of prominence, of a want of moral energy; hair of a...
more than web-like softness and tenuity...
Some critics have commented on the descriptions of both Madeline and Roderick, arguing that they appear incredibly vampire-like, but there is definitely something of the supernatural about their appearance.
Add to this mention of hereditary curses, the doom of the family of Usher and the finale where both twin brother and sister die together so dramatically, you can understand that the "fall" of the House of Usher refers to the end of the line of the Usher family with the spectacular deaths of its two surviving heirs, as well as the actual literal "fall" or collapse of the House of Usher in the last paragraph. It is as if the house is so tied up with the fates of its owners that it cannot survive or escape the fate that has come upon them either, thus emphasising the totality of the evil that has destroyed the House of Usher.