A structure (or record or tuple) is a sequence of items of possibly diferent kind. Each item of a structure is usually called a field. Fields do not have an associated index but an offset respect to the beginning of the structure. Structures are laid out in memory to ensure that the proper alignment is used in every field. The base address of a structure is the address of its first field. If the base address is aligned, the structure should be laid out in a way that all the field are properly aligned as well.
What do arrays and structure have to do with indexing modes of load and store? Well, these indexing modes are designed to make easier accessing arrays and structs.