The keyword begin is used to indicate the start of the executable section of a FUNCTION, method of an OBJECT, PROCEDURE, PROGRAM, property of an object, or is used to delineate the start of a block statement.
program Project1; var(..); begin (..); end.
For a block statement, it delineates the top of the block, and is also terminated by an END statement:
if (..)then begin (..) end else begin (..) end;
A begin must be closed with end.