The bid is the amount you are willing to pay to have your ad displayed. You can use automatic or manual bidding, depending on your payment preferences. Using automatic bidding will optimize your bid to reach your objective. For example, you can choose to get the most website conversions at the best price or to get the most installs at the best price. When using manual bidding, however, you can set the amount a conversion is worth to you.
If you use manual bidding, note that for some optimization goals (page likes, post engagement, website conversions, daily unique reach), Facebook delivers ad impressions to people with the goal of generating your desired outcome at or below the bid you specify. As the Facebook ad system tries to predict who in your target audience will be most likely to take your desired action, your average cost per outcome may exceed your bid. Your average costs should be equal to or lower than your bid. If you are optimizing for clicks (CPC) or impressions (CPM), you will be charged no more than the bid you specify. Max bid is the maximum amount you are willing to pay per click (if bid on a CPC basis) or thousand impressions (if bid on a CPM basis) on your Facebook Ads.