Goto is what you do after a reduce. It's very like a shift, except that it is handlng a non-terminal while shift handles a terminal.
A reduce action removes some items from the stack and uses them to create a non-terminal, which must then be pushed ("shifted") onto the stack. As with a shift action, pushing a non-terminal moves the state machine to a new state, which is why we call these table entries "goto" entries; they specify which state the state machine switches to.
A shift action is looked up in the transitions of the current state. A goto action is looked up in the transitions of the state "revealed" by popping the stack during the reduce action. In both cases, the effect is to move the dot in each of the state's items one position to the right, moving over a terminal (shift) or a non-terminal (goto).