Like a map which shows a diagrammatic representation of an area of land or sea showing physical features, cities, roads, etc.. A sitemap helps visitors and search engine bots easily find all your pages form a single location.

A site map could be of

HTML sitemap - helps visitors navigate your website easily
XML sitemap - helps bots find all your pages easily
While there is no limitation for a HTML site, XML sitemap file is limited to 50MB (uncompressed) with a maximum of 50,000 URLs. For that reason, if you have a very big website having 50K urls, you can nest the site maps.

There are many free XML sitemap generators there you can use to create a sitemap upto 500 urls.