Also, what's 1st party and 2nd party content?

a person who is not a party to a contract or a transaction, but has an involvement (such as a buyer from one of the parties, was present when the agreement was signed, or made an offer that was rejected). The third party normally has no legal rights in the matter, unless the contract was made for the third party's benefit.

First party is YouTube, second is you the user, third is generally advertisers, or anyone who pays YouTube to have their content put in front of you. The contract refers to the legal agreements set forth by YouTube. Privacy agreements, user agreements, and the fact that YouTube owns anything you publish there.