Compiled Expression Trees misunderstanding?

I have this expression :

Expression<Func<string, bool>> f = s => s.Length < 5;

ParameterExpression p = Expression.Parameter (typeof (string), "s");
MemberExpression stringLength = Expression.Property (p, "Length");
ConstantExpression five = Expression.Constant (5);
BinaryExpression comparison = Expression.LessThan (stringLength, five);
Expression<Func<string, bool>> lambda= Expression.Lambda<Func<string, bool>> (comparison, p);

//lets : test

Func<string, bool> runnable = lambda.Compile();
Console.WriteLine (runnable ("kangaroo")); // False
Console.WriteLine (runnable ("dog")); //True

I want to ask about the .Compile()

What does it compile ? And what is the difference between the first execution vs later executions...?

Compile should be something that happens once and not happens again later ....

What / How does it help me ?