If you are just starting out with D365 or moving from AX2012 then it helps to have a little orientation on where to find things. If you are coming from AX2012 then you will have a lot of basic concepts that still apply in D365 but there are new ways to do things.
When starting development you will need to create a new model. A model can be used to package up the development we are doing so you can later deploy to an environment. To actually do work within Visual Studio you will need to create a project. A project is like any other Visual Studio in that it organizes the items you are working on.
D365FO 1611 – Update3 (7.0.4307.16141)
If you are starting our with D365 development this is a good help topic page to start with