a--这里不懂是什么意思求大神讲解一下

public void bossreduce(Admin a)
{
Console.WriteLine("老板发现了！很生气！");
a--;
}
}
class Admin
{
public double bonus = 10000;
public int sinere = 60;
public static Admin operator -- (Admin a)
{
a.bonus -= a.bonus * 0.1;
a.sinere -= 1;
return a;
}