Insert

Insert[list,elem,n] inserts elem at position n in list. If n is negative, the position is counted from the end. 

Insert[expr,elem,{i,j,…}] inserts elem at position {i,j,…} in expr. 

Insert[expr,elem,{{i1,j1,…},{i2,j2,…},…}] inserts elem at several positions. 

Insert[elem,pos] represents an operator form of Insert that can be applied to an expression.

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