This is a somewhat difficult question to answer, but I'll try my best to explain. The sentence means that the speaker thought that criminal libel was not a very serious crime, the penalty of which would be to receive a court summons. If a crime requires bail, it means that the offender would be placed under arrest and would require someone to 'post bail' for them—which is paying a certain amount of money to temporarily get out of jail while you wait for your court date. If they are 'summonsed', it means they are just served with a court summons, and rather than being arrested and put under bail, they would be free to live where they wanted while they waited for the court date. A severe crime like homicide would require someone to post bail, while something like shoplifting would only receive a summons. I should also say that 'summonsed' is a word that is not used often in English outside of the legal profession.