What exactly is an ifconfig dropped RX packet?

Packet Dropped seen from ifconfig could be due to many reasons, you should dig deeper into NIC statistics to figure out real reason. Below are some general reasons

• NIC ring buffers getting full and unable to cope-up with incoming bursts of traffic
• CPU receiving NIC interrupts is very busy and unable to process
• some cable/hardware/duplex issues
• some bug in NIC driver

Look at the output of

• ethtool -S wlan0
• iwconfig wlan0

and the content of /proc/net/wireless for any further information.