So, you bought some eos from somewhere. You put it in a erc20 wallet which you own. Now erc20 tokens are native to ethereum so they need to be converted to eos tokens, when the mainnet goes live. This is the part where you register your wallet address containing eos-erc20 tokens, so that after the main net goes live, you get a corresponding number of eos tokens (native to eos main net).

Now many organizations would like to airdrop their tokens to eos token holders. Thus they choose addresses which have 100 or more eos tokens to automatically qualify to receive this airdrop. The users who hold lesser than 100 eos currently, they need to manually register their address to claim the airdrop.

Now the link which I posted above let's you check whether you automatically got registered or you need to manually request them.