Try Catch Throw

I'm trying to understand how I'm going to use Throw in my code. I have a MainForm class to handle the Windows Form GUI and then I have the Manager class to read and save data from/to files.

I use Try/Catch in both classes, but my instructor want me to use Throw in the Manager class and despite that I'm reading about it, I don't get the point what it will do? Will Throw affect the Try/Catch in the MainForm class?

I also use a message box in the manager class if an exception is catched, but no message box are allow to be in the manager according to the instructor, so how would I do then? Can I use the message box in MainForm class only? Preciate some help to understand and expand my knowledge! Thanks!

MainForm class:

try
{
     motelManager.SaveToFile(file);
}
catch
{
     MessageBox.Show("Ett fel uppstod!", "Varning!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
}

Manager class:

 public void SaveToFile(string filePath)
 {
     try
     {
         string newFilePath = filePath.Replace(".bin", "");
         filestream = new FileStream(newFilePath + ".bin", FileMode.Create);
         BinaryFormatter b = new BinaryFormatter();
         b.Serialize(filestream, animals);
     }
     catch(Exception ex)
     {
         MessageBox.Show(ex.Message, "Varning!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
     }

     if (filestream != null) filestream.Close();
 }