The phrase "to directly cause the required threshold of harm" means that there is a certain level of harm/violence/injury,etc that has to be done before the ICRC considers it to have reached the level of importance for them to take action or to note that action.
The rest of the phrase simply defines the harmful action as being undertaken by one group (the belligerent) and injuring another. The "nexus" word simply means that the two groups are connected in that action.