Using the Scroll to element trigger in your Event means that the Event will be triggered when an element is visible in the screen.

This trigger will let you choose an element from the site by using the On-site widget.

You can also specify whether it should be entirely visible or not by switching to Advanced mode. The default is that it doesn't have to entirely visible. 

You can however select "entirely visible", it means that your Event will be triggered only when the whole element is visible in the user's window.

An Event using the scroll to element trigger is actually triggered as soon as the element is visible, which means that it can sometimes be triggered when the page loads, if the user has a high screen resolution and doesn't need to actually scroll to see the element.

The scroll to element trigger is almost always used with the Restrict per visitor or session condition. If it is not used with that condition, the Event may be triggered very often, since it will be triggered on every scroll action.

Further reading:

How to install the onsite widget?