From the datasheet:
8.5 Brown-out Detection
ATmega48P/88P/168P/328P has an On-chip Brown-out Detection (BOD) circuit for monitoring
the VCC level during operation by comparing it to a fixed trigger level. The trigger level for the
BOD can be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure
spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as
VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.When the BOD is enabled, and VCC
decreases to a value below the trigger level (VBOT- in Figure 8-5 on page 49), the Brown-out
Reset is immediately activated. When VCC increases above the trigger level (VBOT+ in Figure 8-5
on page 49), the delay counter starts the MCU after the Time-out period tTOUT has expired.