This rule primarily (but not solely) covers foreign trading sites and general forms of advertisement (of your business, for example). Never post these. There are a couple of regular sites that can be posted, however. To give you an idea, people sharing their Twitch streams is accepted, provided it's not done excessively and in the right places. Rather harmless stuff like news sites or game info pages are typically tolerated. Just no actual advertising or linking to harmful material.