It means you have extended your layers into negative time which you can easily verify by opening the layer view and checking the time code.
Thanks you Mylenium,
Do you also know when should you be aware of extending your layer into negative space? I can't see that it currently affects anything?
It shouldn't matter for any still layers, it just looks confusing. It's usually only relevant when using motion stuff or features like time-remapping. not so much that it technically is problematic, but you simply lose track of what the actual "time" is for a given element, e.g. in a pre-comp.