The zero lower bound (ZLB) is a monetary policy situation where the central bank has lowered its policy interest rate to zero or near-zero and can't cut further using conventional methods.
Normally, central banks cut rates during recessions to make borrowing cheaper and stimulate the economy. But once rates hit zero, this tool becomes useless.
At the ZLB, central banks must resort to unconventional policies like quantitative easing (buying government bonds to inject liquidity) and forward guidance (clear communication about future policy intentions).
The ZLB can make economic recovery very difficult. Japan, the Eurozone, and the US after 2008 all spent extended periods at the ZLB.
Bangladesh doesn't typically face ZLB conditions, as Bangladesh Bank maintains its repo rate at relatively high levels.