You may find it helpful to show the Rulers in the Project panel. While the Project panel is in focus, choose View > Show Rulers.
Pixels are the units for X and Y coordinates of the Source Clip and the Sequence.
Each Source Clip has an Anchor Point that by default is at the center of the Source Clip's frame size. 0, 0 is the upper left corner and the X value increases from left to right and the Y value increases from top to bottom. So, if the Source Clip is 1920-by-1080, then the Anchor Point defaults to 960-by-540 (960 pixels form the upper left corner and 540 pixels down from the upper left corner).
The Position value is where the Anchor Point is located in the Sequence measured in pixels from the upper left corner of the frame which is also 0, 0. By default, the Anchor Point is positioned at the center of the Sequence frame size. So, if the Sequence frame size is 1920-by-1080, then the Anchor Point of the Source Clip is positioned at 960-by-540 (960 pixels form the upper left corner and 540 pixels down from the upper left corner).
To animate a frame from left to right, the first Position key frame would be -960, 540 and the second Position key frame would be 2880, 540.
An easy way to position footage is to double-click it in the Program panel while the Selection Tool is selected. While the Clip is selected in the Project panel, click and drag anywhere inside of the handles. Clicking and dragging the handles along the sides is a Scale change. Hovering over the Anchor Point at the center and then clicking and dragging is an Anchor Point change. So, for Position changes, make sure you click somewhere between the handles on the sides and the Ahnchor Point at the center.
If you need to reset anything, click the Reset Parameter button to the right of each parameter name in the Effect Controls panel.
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