DotNest is an Orchard SaaS provider where you can host both Orchard Core and Orchard 1sites: Orchard Core is a feature-rich, mature, very flexible content management framework (it's not only a content management system but a very powerful software development framework too) built on .NET Core and later. It has a relatively steep learning curve for developers but once you get to know it you'll be able to develop applications with it rapidly. You can host Orchard apps yourself or you can use DotNest to create Orchard sites with a click of a button, see "What is DotNest?".
If you don't know Orchard (enough) yet, take a look at Orchard's home page, especially the official documentation. If you just want to quickly give Orchard a test drive without even creating a site on DotNest take a look at Try Orchard Core.
There are also many third-party tutorials out there: for beginners, check out our Dojo Course 3 with its tutorial videos (while the Dojo Course starts with using Orchard from the UI, it is mainly aimed at developers).