When I double tap (Not press) the icons or what ever your screen is on gets pulled down half way
Why does it do this?
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So you can reach them when holding the phone with one hand.
Apple refers to this feature as reachability. This allows you to reach apps at the top of the screen with your thumb.
If you don't like this, change the setting in that app under General > Accessibility - it's towards the bottom just under Home Button