It is when you judge that somebody is responsible for a (usually negative/ serious/ damaging) situation/ event. e.g. The bad financial situation of the company may be partly a result of the the slowing of the economy, but the CEO must also be held accountable. The collapse of the building was certainly due to structural faults. So it is the architect who must ultimately be held accountable. An official enquiry into the terrorist attack must be set up, in order to determine who will be held accountable.

Hi, this means a person taking responsibility for their actions or commitments.

Hold somebody responsible or liable