Dwell Control is how users of eye- or head-tracking assistive technologies control how they click, drag, and scroll items on the screen. Dwelling is holding the mouse still for a period of time which then triggers and action like a mouse click.
The setting is somewhat hidden, but can be accessed by going to:
Apple menu > System Preferences. Select Accessibility, then Keyboard Control>then Accessibility Keyboard pane. The “Dwell Options…” Button is toward the bottom right. Hint: a faster way is to type “dwell” in the search box.
Depending on how you use dwell, you may want to keep in the menu by selecting Show Dwell actions in menu bar
Additional details on the items of the pane can be found in Settings pane of Dwell Control Accessibility System Preferences on Apple’s support pages