In the timeline view, you can select multiple lines by maintaining the Command or Shift key of your keyboard and click on the tasks lines.

When using the Command key and clicking on lines, you can individually add/remove them from your selection.

When using the Shift key, select the first line, and select the last line of your desired selection. All lines in between will be selected.

Once you have selected multiple lines, you'll see an action selector appearing at the bottom left of your screen.

Depending on the items you selected, the possible actions may change. You can, for example:

Managing & Organizing Tasks