would anyone please tell me the meaning of this word?
Adults who were rated optimistic recovered more quickly in the short and long run after operation and had fewer hospital readmissions than pessimists.
If somebody is admitted to hospital, it simply means that they go/are taken there for treatment. If the treatment proves unsuccessful after they have left (been discharged), then they have to be readmitted. The words that I have coloured red could therefore be paraphrased as had to return to hospital for further treatment fewer times,