In addition to what Vipul said, seed addresses can be addresses that you control and considered as "test profiles". You can use these addresses in your deliveries to control reception of the messages and possible fraudulant uses (for example by insterting a typo in your name on purpose, you will know where it can come from if you receive other communications with that same typo).
Seed addresses are the mechanism to add recipients to final target whom you wish to deliver an email no matter if they are eligible for targeting criteria.
Let's say you wish to send an email to everyone whose age is less than 30 but then you also wish to send the same email to few other recipients whose age is greater than 30, then you can add them as seed addresses.
A better use comes in the case of Direct email. Direct emails are sent by post and hence to ensure the service provider is sending the postal mail to everyone on your target list, you add a few of your known people as a seed to this list.
If an email is received through post by your contact it can be assumed that your service provider is doing its job.