The Width property defines the thickness of the line/outline. As with the other Line properties, you would usually set the Width on the Line tab of the Polyline Properties window but you can also animate it if required.
You can enter an expression to animate the Width so that it defines a specific thickness or so that it changes thickness according to the value or state of an item in the database.
For the expression, you should enter the size of the line you require (the number you enter is recognized as being a point size). For example:
IIF( ".1 Bit Digital.CurrentState" = 1, 10, 5 )
This expression means that when the point named '1 Bit Digital' has a current state of 1, the line is 10 pt thick, and when the point is in state 0 the line is 5 pt thick.