Snippets are places in parts of a web page, they allow the user edit content on a page that is not in the main body of content. e.g. a slogan may be shown at the top of a page.
The snippets can also include javascript, e.g. you can include the tripadvisor javascript widget. ( You should switch the WYSIWYG editor to HTML when adding javascript )
Snippets can be customised for each language your website is translated into. When creating or editing a snippet on a site that has multiple languages each language will have a tab to enter content for that translated version of the site.
You must tick the box beside each translation you wish to use, any content entered without ticking the corresponding tick box will cause the information not to save for that language.
When visiting a translated version of a site, the website will check for a translated version of each snippet, if it doesn't exist ( i.e. the tick box was not selected ) then the snippet for the default language will be used. Should you want to prevent the default language snippet from being shown but do not want to show translated content you should tick the translate check box but enter no details in the content area.