Extract

Extract[expr,pos] extracts the part of expr at the position specified by pos. 

Extract[expr,{pos1,pos2,…}] extracts a list of parts of expr. 

Extract[expr,pos,h] extracts parts of expr, wrapping each of them with head h before evaluation. 

Extract[pos] represents an operator form of Extract that can be applied to an expression.

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