To understand this, you need to look at what is said before and after. Young Siward is saying that there is really not all that much fighting going on. Malcolm says that Macbeth's men are fighting as if they aren't really trying to hurt Malcolm's men.
So what is going on with the part you cite is that Macbeth's men must be fighting among themselves. Some of them must have gotten sick of Macbeth and have started fighting to try to overthrow him. This shows how Macbeth's bad deeds have come back to haunt him.
The concept of 'division' is a big theme in the play 'Macbeth' by William Shakespeare - division of realm, division of country, division of Scotland, division of marriage, divsion of clan, division of friendship and division of family. Some enthusiasts of the division argument even go on to hypothesize that the body part ingredients of the cauldron illustrate the division and partition of English/Scottish society at that time - others are keen on the divided mind aspect of Macbeth. Siward even notices the men changing sides and changing their minds - not uncommon in a tribal hierarchy where some men would fight for the highest bidder much like mercearies. All show a divided society in turmoil.