This is the width or height of the invisible window that fvwm creates on the edges of the screen that are used for the edge scrolling feature.

In order to enable page scrolling via the mouse, four windows called the "pan frames" are placed at the very edge of the screen. This is how fvwm detects the mouse's presence at the window edge. Because of the way this works, they need to be at the top of the stack and eat mouse events, so if you have any kind of error along the lines of: "mouse clicks at the edge of the screen do the wrong thing" you're having trouble with the pan frames and (assuming you do not use the mouse to flip between pages) should set the EdgeThickness to 0.

A value of 0 completely disables mouse edge scrolling, even while dragging a window. 1 gives the smallest pan frames, which seem to work best except on some servers.

2 is the default.

Pan frames of 1 or 2 pixels can sometimes be confusing, for example, if you drag a window over the edge of the screen, so that it straddles a pan frame, clicks on the window, near the edge of the screen are treated as clicks on the root window.