To start selected tasks at the same time press 
on running toolbar.
Start selected tasks
immediately at program startup - tasks may be started automatically at program startup. You should also set "Start automatic tasks immediately" at Options - Schedule or in main menu "Tools - Automatically run tasks"
when Run buton is pushed - these tasks will be only started when you run them manually.
on ... - you can schedule these tasks. They will be started at specified date and time. When a scheduled task cannot be started at defined time, AbleGet may during next program start the task automatically or ask you. See Options - Schedule.
Running tasks will not be stopped. Each task can be also scheduled individually.