This effect makes the Rock be replaced by a Pebble, which is a 1/3 monster for 1G that also has Taunt. (Its Cost doesn't matter much due to being directly summoned once Rock dies.)
This essentially means that you are protected from at least two monster attacks, unless the Rock gets silenced. However, this does not mean that you should replace Jerry as he is cheaper and enough gold can be earned for him on the start of your second turn, whereas the Rock will require saving up for at least the third turn.
It is also recommended to save the Rock for moments where two taunts are needed/there are multiple enemy monsters on the field (typically after turn 7). Because of this, Jerry is possibly suited towards quicker decks and the early to mid game unlike Rock.
Keep in mind that the Pebble cannot attack right away if you kill your Rock during your turn. It can be used right away if the opponent kills the Rock, but not the Pebble.