Basically, the Online Pass is pretty self-explanatory:

If you want to enjoy the online portion of a game, you either have to purchase the game new (new copies include a code for the Online Pass), or you have to buy the Pass separately. But what business does the Pass have in dictating certain parts of the single-player experience?

According to Game Informer , players will have to have a Uplay Passport if they wish to enjoy the Kenway's Fleet mini-game in the campaign. Kenway's Fleet lets you send assassins-in-training on various missions in the hopes that they bring back some loot. It's a variation of past mechanics that have allowed your assassin prodigies to earn experience. It's all about ships in ACIV.

So why is it only available to those who have a Passport? Well, basically because if your Friends are also playing the game and tackling missions, missions you undertake can be completed faster. It's not a bad idea but in my eyes, it's just another example of so-called "connectivity" invading the single-player space. For my part, I've never activated the Uplay Passport in any AC, 'cuz it's useless to me, and I don't feel like having my progress tracked for any reason.

Now, it seems I'll have no choice if I want to experience all that Black Flag offers…in the single-player offline mode . Whatever.