Change groups are the codes that mark how a TAF forecast evolves. BECMG means a gradual, lasting change; TEMPO means temporary fluctuations lasting in total under half of the stated period; FM means a complete change of conditions from a given time onwards.
Time groups are written as day plus hour in UTC: 1022/1024 means the 10th of the month, 22:00–24:00 UTC. These groups answer the question that actually matters: does the bad weather fall inside YOUR flight window, or does it pass by outside it.