What does it mean when I get a can't fragment error?

Receiving message Can't fragment

DF bit is set by the application  This error (type 3 (destination unreachable), code 4 (fragmentation needed but don't-fragment bit set)) is returned by a router when it receives a packet that is too large for it to forward and the DF bit is set. The packet is dropped and the ICMP error is sent back to the original host. Normally, this tells the original host that it needs to reduce the size of its packets if it wants to get through. Recent systems also include the MTU of the next hop in the ICMP message so the source knows how big its packets can be. Note that this error is only sent if the DF bit is set; otherwise, packets are just fragmented and passed through.