What does summary dismissal mean?

I was due to attend an investigation meeting last week for alleged gross misconduct namely incorrect use of gift cards purchased for charity and as i did do what they are investigating into I didn't attend and submitted a sick note and my resignation. In my resignation I asked them to accept my resignation with immediate affect due to irrevocable breach of trust and confidence from myself towards the company. Over the weekend I received a letter saying that they were unable to accept my resignation and a disciplinary is arranged for this week (today) and at the end says this is a serious allegation which upheld could result in your summary dismissal for gross misconduct. As I said I did do what is alledged and cannot face them as I am very embarred as to what I did, what does it mean in summary dismissal?