What does it mean when a server goes into safe mode on Minecraft?

Been playing the server but when i got on today it was in safe mode and if you moved your character glitched out and then lagged out and gave you the error To many packets? (Extreme lag?). The weird thing is i was still able to throw stuff and eat food and look in my inventory but the second i moved it glitched out then lagged out.So what does it mean? Does it have to do with being in safe mode?