Fields and properties
A class keeps its data in fields. Thermostat has one field, _target, which starts at 20. It has no access modifier, so it is private: only code inside the class can read or write it.
Target is a property. From outside it looks like a field, but it is a pair of methods called accessors: get returns a value, set stores one. Here set uses Math.Clamp to keep the value between 5 and 30.
Run: set and get
Program.cs runs from the top. Line 1, new Thermostat(), makes an object on the heap and gives every field its starting value, so _target is 20. The variable t refers to that object.
Line 2, t.Target = 24, looks like writing a field, but C# calls the set accessor. Inside it, the keyword value is 24. Math.Clamp(24, 5, 30) is 24, so set stores 24 in _target.
Line 3 tries t.Target = 99, far too hot. set runs again with value = 99. Math.Clamp(99, 5, 30) returns 30, so _target becomes 30. The bad number never reaches the field.
Line 4 reads t.Target. That calls the get accessor, which returns _target, 30. Console.WriteLine prints 30. Code outside the class never touches the field itself, only the property.
Kinds of property
Fahrenheit has only a get, written with =>. It stores nothing: every read works it out from _target. In whole-number maths 30 * 9 / 5 + 32 is 86, so line 5 prints 86.
Fahrenheit has no set accessor, so it is read-only. A line such as t.Fahrenheit = 50; does not even compile: the compiler stops with error CS0200. Its value can only change through Target.
Floor { get; set; } has no bodies: it is an auto-implemented property. The compiler writes a hidden backing field and both accessors for you. Use one when no check is needed; write your own field when set must guard it.
What happens
// step 1/9 · field
class Thermostat {
int _target = 20; // field, starts at 20
no public, no private written → class members are private by default
outside code cannot touch _target directly
The program is real C#: put the five statements and then the class in Program.cs, and dotnet run prints 30 and 86. In one file, top-level statements must come before any class. Under the hood the compiler turns the accessors into two methods, get_Target and set_Target(int value), which is why reading or writing a property can run code.