The operators @@ and @@@ are shorthands for two forms of Apply, which is a structural manipulation function. They work on the special way Mathematica expressions are built: they are of the form h[x,y,...], where h is the "head" of the expression. The function Apply replaces the heads of expressions with other heads:
f@@h[x,y,...] gives f[x,y,...] (top level replacement)
f@@@h[g[x],g[y],...] gives h[ f[x], f[y],...] (lower level replacement)
The & operator indicates that what comes before it is a "pure function", whose arguments are indicated with #, or #1,#2,...
It is not uncommon to find & immediately followed by @@ or @@@, but they are distinct objects.