What is a Black Box? How will it solve the mystery behind Gen Bipin Rawat's helicopter crash? Master Stroke

The sudden and untimely passing away of the Chief of Defence Staff (CDS) General Bipin Rawat in a military helicopter crash near Coonoor in Tamil Nadu has raised the big question about how the crash happened.  What is a Black Box?  How will it solve the mystery behind Gen Bipin Rawat's helicopter crash?