The function b[c_] has been previously defined. Does that "1" refer to the smallest number of a list?
If you just want to know what the double brackets mean, you can highlight them and use help. Or, you can use Information (?), for example:
a /. b[c][] //Hold //FullForm
Yes, 1 refers to the first element of the list. "/." is a rule replacement so I presume in this case that function "b" is returning a list of rules, which the line of code intends to use as a replacement operator for a.
Starting list or array notation at "1" is sometimes confusing for people used to "c" starting indices at 0. Note that there is a form of "Mod" that will properly provide the right index so you don't end up with "0" when you try to get a part of a list.