Dead Hash Rate is the amount of shares you are submitting that were stale or DOA. You will want that to be as low as possible. Your rate looks ok. Things that affect your dead hash rate are
latency to the pool and miner settings or problems. Some have commented that lowering their intensity reduced their Stale/DOA rate.
Displaying that statistic is common for p2pools as it helps you determine if you are mining efficiently. I don't think I have seen it on other types of pools.
Edit:
DEAD ON ARRIVAL - Dead shares were too old by the time they arrived at the pool. Very high dead rates can indicate miner misconfiguration.
Q: "Do the 'orphan' and 'dead' shares in P2Pool's status display hurt me?"
A: They shouldn't - It's normal for some fraction of everyone's shares to end up orphaned or dead. Because payouts are calculated by counting how many shares you have relative to others, everyone with normal configurations is equally "hurt" by this. However, if you have a large proportion of stales, your payout will be hurt.
More can be read at:
https://en.bitcoin.it/wiki/P2PoolPlease take note that a lot of what is mentioned is specific to the larger Bitcoin p2pool but the same concepts apply to altcoins...just at a much smaller scale.