The "step number" is a button. Hovering over it reveals that it's used to move that particular step to a new position in the cadence. Click it to bring up the window below.
The "Move Step" window allows you to choose a new position for that particular step, it will be highlighted by default. Use the handle on the left side, next to the step number, to drag the step to a new position.
Once you've decided on the new position, read the confirmation message "Step X will become Step Y" to ensure your desired outcome. Hit "SAVE" to bring up the confirmation window.
The confirmation window will only appear when changes are made to a live cadence. It's here to inform you that the prospects currently enrolled in the cadence will be affected by this change. If that is your desired outcome, hit "CONFIRM". Give the system a few seconds to make the changes, and you're done!
Inserting a new step
To add brand new steps in the middle of a cadence, find the '+' button between current steps on the cadence timeline. Click it to add a blank step, as shown below.
From this point, you may configure the blank step to your wishes. Once you finalize the type of step and the messaging, the system will ask you for confirmation, similar to the reorder step operation. Confirm the changes and allow a few seconds for them to take effect.