First Page Bid is the bid that you likely need to set for your advertisement to be shown anywhere on the first page of search results.
This estimate approximates what cost-per-click (CPC) bid is needed for your ad to show anywhere on the first page of search results when a search query matches your keyword. Your ad can still appear if your bid does not meet this estimate, but it's less likely to appear on the first page of search results.
In Google AdWords, the estimate is based on each keyword's Quality Score and competition from other advertisers. If your first page bid estimate is very high, it may mean that your keyword's Quality Score is poor and could be improved.
To see first page bid estimates for your keywords, find the "Est. first page bid" column on the Keywords tab in the AdWords interface. The estimate isn't a guarantee -- on occasion your ad may not make it to the first page, even when you meet the first page bid estimate.
The opposite may also occur. Even when Google tells you your bids are too low to appear on the first page, you may find average ad positions between 1 and 3. This may be explained by keeping in mind the continuously changing environment of the AdWords auctions, which is a large part in Google's First Page Bid predictions.