بعضهم البعض ( ba3duhum alba3d)
means something like "each other"
examples:
أحَبُّوا بعضهم البعض ( ahabbu ba3duhum alba3d) they loved each other
كانوا يضربون بعضهم البعض ( kanu yadribuna ba3dunhum alba3d) they were hitting each other.
hope I could help :)