Let me expand upon that just a little bit to hopefully help qumqats understand what you mean.  Finding shares in p2pool is equivalent to solving a block.  Your miners, when they find one, are announcing to the p2pool network that they have found a low enough hash to satisfy requirements and are thus adding a new block to the chain.  An orphan share means that somebody else also found a solution that satisfied the requirements and announced their solution to the network.  Unfortunately for you, their solution was accepted and added to the chain prior to yours.

You're not completely out of luck, though, because your solution might also satisfy the Bitcoin network's requirements.  In that case, your orphaned share would solve the BTC block, and everyone gets their share of the reward.

Take a look at the following contrived example to see how it might work:

You find a share of difficulty 10,000,000,000
Another miner finds a share of difficulty 1,000,000,000

Both of you submit your work.  His is accepted as a share and yours becomes orphaned.  Yours is, however, accepted by the BTC block chain as a valid solution.  The block is added to the chain, 25BTC are generated and distributed to the miners in p2pool, along with transaction fees from the generated block.

Dead shares work similarly, except for the fact that they never make it out of your local environment.  Whereas an orphan share means you announced to the p2pool network that you found a solution, a dead share you don't even bother announcing because you have already been informed by the network that the share is invalid (i.e. the block height is lower than the current block being mined).  Just like with an orphaned share, your solution might be a valid solution to the BTC network, and if so, it solves the block and everyone gets paid.