Part

expr[[i]] or Part[expr,i] gives the ith part of expr. 

expr[[-i]] counts from the end. 

expr[[i,j,…]] or Part[expr,i,j,…] is equivalent to expr[[i]][[j]]…. 

expr[[{i1,i2,…}]] gives a list of the parts i1, i2, … of expr. 

expr[[m;;n]] gives parts m through n.

expr[[m;;n;;s]] gives parts m through n in steps of s.

expr[["key"]] gives the value associated with the key "key" in an association expr.

expr[[Key[k]]] gives the value associated with an arbitrary key k in the association expr.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.