Difference Action = drop and Action = reject Mikrotik

The use Action = drop

If you choose to use the option Action = drop, then the data coming from the client will be discarded (drop) by the router. This is done in secret, with no rejection message sending ICMP (Internet Control Message Protocol). So if we send a ping message from CMD, then the result is Request Timed Out (RTO). 

When using the option Action = drop, then the ping process will display the message "Request Timed Out" as shown below:

Action = reject the use of

As for the option Action = reject, the data packet is discarded by the router but the router will provide rejection message packet by sending ICMP rejection message. You can choose what message would be sent if using the reject option.

When you try to ping then the result is "Destination net unreachable" Reject the same as with the previous option, as shown below: