Oh, you mean the icons that appear besides the stack on the map?
I think on the manual most of them are explained also.
The one you ask for is probably the entrenched icon. it means the stack has been there for a turn or more and the units have now some defensive advantages.
There are two: a little men behind a parapet (level one) and a little gun sticking out a parapet (level wo entrenchment).
The level tow can only be attained if the stack has artillery.
To confirm this, check the icons on the upper right of the units panel. By the supply, activation, detection… icons, the entrenched stacks will have a zigzagging line icon with a number (1 or 2).
Cheers!
EDIT: now that i think of it, it can be also the "inside structure" icon.
