What are meanings of fields in /proc/net/dev?

You can have a look at net/core/dev.c in the source tree to see what it means:

seq_printf(seq, "%6s:%8lu %7lu %4lu %4lu %4lu %5lu %10lu %9lu "
       "%8lu %7lu %4lu %4lu %4lu %5lu %7lu %10lu\n",
       dev->name,
       stats->rx_bytes,
       stats->rx_packets,
       stats->rx_errors,
       stats->rx_dropped + stats->rx_missed_errors,
       stats->rx_fifo_errors,
       stats->rx_length_errors + stats->rx_over_errors +
        stats->rx_crc_errors + stats->rx_frame_errors,
       stats->rx_compressed,
       stats->multicast,
       stats->tx_bytes,
       stats->tx_packets,
       stats->tx_errors,
       stats->tx_dropped,
       stats->tx_fifo_errors,
       stats->collisions,
       stats->tx_carrier_errors + stats->tx_aborted_errors +
        stats->tx_window_errors + stats->tx_heartbeat_errors,
       stats->tx_compressed);

So:

• receive errors means any kind of invalid packet, e.g. invalid length or invalid checksum
• transmit errors are
  • carrier errors
  • aborted errors
  • window errors
  • heartbeat errors
    (whatever they all mean)

And yes, I think drops means when the device dropped a packet because it ran out of buffer space.