Interesting question! According to the page linked below, they are Restricted Braking Distance indicators. Presumably to signify a specific braking distance, but I don't know the details.
Could they be related to the pedestrian crossing at the end of the station platform to help prevent overshoots or perhaps something to do with the short signal block, maybe?
It looks like there is also a 3rd sign in this category which is specific to freight trains. Hopefully someone more knowledgeable will be along to help soon!
Marked W19 and W20 in the image...
http://pkprepo.net/railway-signs-of-polish-railways/
Thistle Whistle's post in DG entry signal indicators was marked as the answer