The Original and Remaining lag fields are manually entered when it is known that a resource will not be available or is not needed when the activity starts.

By typing in a lag, you delay the start of the resource by that amount of lag.

Original Lag is what was typed in and is used when there is no progress on the activity.

Remaining Lag is what lag is left after the activity has started. Remaining lag is automatically reduced once the activity is started and the Data Date is moved forward.