[Bro] What does PacketFilter::Dropped_Packets mean?

[fasf safas]

Hi,
i don't understand very well what PacketFilter::Dropped_Packets means.
For example, i've found a notice like this:

2148880 packets dropped after filtering, 2233772 received, 2233380 on link

Can you explain me better the meaning of these values?
Thanks
Fab
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