What is the difference between a Solution and a Project in vb.net?

A solution is a container for projects. A single solution may contain zero or more projects. Loading the solution file will load all the associated projects. If you are using project references between the projects in the solution (one project depends on another) opening only the project file might not compile as it won't be able to compile the referenced project.

The concepts are visual studio concepts and are not specific to VB.NET.

A solution can contain several different projects.

If you load a solution that contains several projects, they will all load and you will be able to compile them as one unit. You cannot do this if you open a single project.

See this article - Structuring Solutions and Projects for in-depth guidance on how to use solutions and projects.