The grape press is the means of extracting grape juice from the grapes.

In white wine production, pressing usually takes place  immediately after crushing and before primary fermentation.

In red wine production, the grapes are also crushed but pressing usually doesn't take place until after or the near the end of primary fermentation so that there is an extended time in contact with the grape skin to increase tannins and colour and other phenolics from the skin.

Approximately 60% of the available juice is released from the grape berry during the crushing process (called free run juice)  and doesn't require the use of the press.  Free run juice is the best quality and balanced juice, and over pressing should be avoided as over pressed  juice can be more astringent.