This simply means that the term your employer breaches must be so serious it justifies you resigning and bringing a claim for constructive dismissal. For example, failure, or a refusal, to pay commission due and owing for a month would be grounds to resign and claim constructive dismissal. It may be the case that there have been a number of smaller incidents, such as where there has been bullying, and that over time these incidents built up and eroded the relationship, the final incident being ‘the straw that broke the camel's back' which leads to the constructive dismissal claim.