A dynasnip, as they were in the original vanilla implementation, is a bit of dynamic content that you can include into pages.
vanilla-rb tries to generalise this a bit, blurring the division between normal snips and dynasnips. A dynasnip is simply a snip whose content, when rendered, is dynamic. So, the snip contents may be Ruby code, and could be rendered by the Ruby renderer, resulting in content that changes based on the parameters given to the renderer.
Read more about the concept on the vanilla-rb page.