You can pause individual Sends in a Flow or Pause the entire Flow.
To activate the Flow, use the toggle at the top of the screen. If you wish to pause it, no further sends in that block will be effected.
You can also set the entire Flow to Active/Inactive (On/Off).
If your Flow is paused, everyone already in the Flows stays in whatever step they were in when the Flow was paused, and will remain there until the Flow is unpaused. Once you unpause a Flow, the process will resume and triggers/filters/delays and sends will begin processing.