That is a expression bodied property. It can be used as a simplification from property getters or method declarations.
From C# 7 it was also expanded to other member types like constructors, finalizers, property setters and indexers.
Check the MSDN documentation for more info.
"Expression body definitions let you provide a member's implementation in a very concise, readable form. You can use an expression body definition whenever the logic for any supported member, such as a method or property, consists of a single expression."