What does Article 11 of the declaration of human rights mean?

Article 11 of the declaration of human rights says that:

• Everyone charged with a penal offence has the right to be presumed innocent until proved guilty according to the law in a public trial at which he has had all the guarantees necessary for his defence.
• No one shall be held guilty of any penal offence on account of any act or omission which did not constitute a penal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the penal offence was committed. 

Further Reading:

• Universal Declaration of Human Rights
• Human Rights Day