Ethnic tension is a term referring to tension between different ethnic groups. This can be caused by many different factors, all of them various social inequalities, ranging from a lack of wealth to intentional malice. If left untreated, it may start an ethnic war.
In short, some level of ethnic conflict, ranging from fairly peaceful to extremely violent is the norm, not the exception in the modern world.
Also, there have been cases in which relatively severe ethnictension, even warfare, have been resolved, or are being resolved through compromise and reconciliation.
Malaysian ethnic relations are also far more harmonious than they were 30 years ago when it seemed as if the Chinese and Malays were headed toward outright ethnic warfare.