Style is the visual appearance and format of the HTML page. Its role is not only to makes pages appear visual attractive but also to make them friendly for different screen sizes.

Without style, HTML pages would look boring and you would have a hard time trying to access them through your phone or tablet.

The language of style is CSS which stands for Cascading Style Sheets.

In general the visual output of the web page is the combination of HTML (structure) and the CSS (style).

Let's see some examples:

Your "Choco Home" page without style

Your "Choco Home" page with "Modernist" style

Your "Choco Home" page with "Oldstyle" style

Your "Choco Home" page with "Ultramarine" style

The above style sheets are ready-made examples provided by W3C.  You can find them here