The emails are just telling you that your router has recieved the thresh hold number of data packets which trigger the DOS alert and thus sends you an email about it. This may or may not be a true DOS attack as such but the defination of a DOS is academic really its not that important. What is important is that your router and (its firewall assuming you have one built into it) its configuration is blocking or dropping these inbound packets. i.e. This proves your router and or firewall is working well and as intended. Keeping everything behined your router safe (PC or Internal network,etc).
DOS is just a tidal wave of data packets which can be sent form one source or mutiple sources (using infected machines) to one website or webserver and the usual motive is to overwhelm the server so it grinds to a stop or falls over completely. There is a new type of spamming virus that is designed to do just this if it can infect and ADSL i.e. always on machine connected to the web. Good AV products such as Norton AV should catch these though and prevent outbound packets if suspected DOS is attempted.