The term "zoom level" refers to the spacing of bars on your UA charts. If you zoom in, fewer bars are spread across the screen for a close-up view of short-term activity. If you zoom out, more bars appear on the screen for a more compressed, longer term chart.
You can adjust the zoom level on your 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.
Use the "Save Current Zoom Level" feature on the right-click menu to record the current zoom level and have it applied automatically to all subsequent charts during the current UA session. Simply create a chart, set the zoom level, click the right (not left) mouse button and select "Save Current Zoom Level."