Control flow is the collection of data flow task which is provide structure , functionality in package and connect the executables , container, task into a ordered control flow. SSIS provide three types of control flow containers ,task, precedence containers.
Container provide structure in package ,task Provide functionality and precedence containers responsible to connect executables , and task into rank, priorities or ordered control flow.
A simple task has been represented in above pic, SSIS First Task is reading xls file from a destination it's execute successfully then it will execute FST-Move File it is responsible to move xls file from destination to a given archive file. I f it failed then it will follow SMT-Failure mail, SMT-Failure mail responsible to send a failure to given mail id.