The gerundive (-ndus, -nda, -ndum) with a form of sum means "must be done, is to be done"; the person who must do it is in the dative.
The gerundive is a verbal adjective made from the present stem with -ndus (cavendus, agendus, venerandus, miserandus). It agrees with its noun: hallucinatio cavenda est "a delusion must be guarded against"; minutissima quaeque … sint agenda "even the smallest things must be done".
The person who must act is in the dative, not with a or ab: mihi agendum est "I must act". Neuter and impersonal, it means "one must": vivendum est "one must live".
est or sunt may be left out: quae … Dii agunt, … veneranda "what the gods do (is) to be revered". With non it means "must not" or "need not"; let the sense decide.
Some gerundives are plain adjectives: horrendus "dreadful", contemnendus "contemptible", venerandus "venerable".