Big ends are refering to the bearing shells around the crank.

The little ends are refering to the bearing shells around the piston.

With poor maintanance and irregular oil changes these bearing shells can wear out, there is then movement in the con rod against the crank causing the knocking noise you here. Usually the only effective solution is either replace the crank and bearing shells or have the crank re ground and fit a slightly bigger shells in.

HTH