Hi @vincent and @MinewireNetwork, the function is called smooth scrolling or nice scroll 
This is not built in to Webflow, but you can add a simple jQuery plugin that can do this: http://areaaperta.com/nicescroll/.
There is also a cdnjs repository that you can link to for the script code:
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery.nicescroll/3.6.0/jquery.nicescroll.min.js"></script>
This would go in the Footer of the site, using custom code: Custom code in head and body tags | Webflow University
There may be other custom code options as well for something like this 
I hope this helps, cheers, Dave