Well, the idea is that fearing battle means you fully understand the dangers and wont put yourself heedlessly in dangerous situations. Tousen has problems with Kenpachi for that reason, since he practically throws himself into battle. He also said you should fear your blade, though the reason for that is slightly different. That is more about control, and the ability to fully understand your blade. Understanding a tool is the only way to make sure it's used effectively, and understanding that that tool is deadly and should be feared is necessary, by Tousen's analysis.
It's not necessarily something I agree with, especially considering it's only his sentiment and none of the other captains have expressed anything similar, but it does make some sense.
