The "Zoom Level" is the spacing of bars on your UA charts. A zoomed-in chart has fewer bars that are spread across the screen for a close-up view of short-term activity. A zoomed-out chart has more bars that appear on the screen for a more compressed, longer term chart that may include significant overlap.
The number of bars that appear initially on your charts is set through "Default Zoom Level" on UA's Charting Preferences screen.
You can adjust the zoom level on any active chart by clicking "zoom in" and "zoom out" from the Chart Tools menu, or by clicking the "Zoom In" and "Zoom Out" icons on the toolbar. Another way to adjust the zoom level is to use the "Zoom by Box" feature on the Chart Tools menu. After making any changes to the zoom level using the above techniques, you can return to the original chart presentation with this feature.
To use, click anywhere on your chart, and then click the right (not left) mouse button. Choose "Restore Zoom Level" from the menu.