Just like you have delete comments to deal with, from time to time you will also accumulate spam comments. These are comments that are not true and helpful comments from your readers but ones that are nonsense or are just being used to get links back to someones own site. Its a good idea to have a spam blocker like Akismet installed to catch these comments ant to get rid of them. But even with a spam blocker some will make it through to your site, so its a good idea to go through and manually delete these comments.
Your homework now is to go through the comments and to get rid of the ones that are not really helpful and real.