What is disclosure?
The act of making something obvious. Disclosing (divulging or explaining), the purpose and interests of a blogger in his/her published posts: written, audio or video.
Why is it important in blogging?
Because advertisers are offering bloggers payment or gifts to create content about specific products or services, the blogger's interest in the product or service may be in part the financial gain. Whether the blogger would have included content in his/her blog about the product or service without gift or payment, the fact is the blogger is receiving payment for certain content.
By disclosing the purpose of a blog, bloggers are letting readers know more about the information they'll be reviewing. Bloggers retain the freedom to write original content, as well as select which advertisers they will represent in exchange for gifts or money.
Any ethical concerns will remain where they've always been - on the individual level. Because it is a blogger's freedom to select which topics will bring them payment, he/she remains responsible for his/her own reputation.