First things first, the pulp of almost all wine grapes is colourless. The skins, however, are richly pigmented, containing a range of compounds such as anthocyanin and tannins that are important contributors to the colour and structure of red wines.

There are many different ways of making red wine, but a common theme to all is the goal of extracting these colour and flavour components from the skins without extracting too much: a common analogy used here is making a cup of tea, where the goal is to take out just enough flavour from the tea leaves, but not letting it stew too long and becoming bitter.

Once fermentation of the grape juice begins in the red wine vats, the carbon dioxide that is naturally produced lifts the skins and forces them to the top of the vats. These lifted skins forms what is called the cap. 

Hand-plunging is the process of forcing the skins back down into the vats to ensure that all of their colour and tannin are infused into the wine. It also evens out the temperature and prevents wine spoilage. Just like the tea, gentle extraction is (most often) best.