Google AdWords allows you to chouse your preferred ad position ranking on the Google search network. For example if you notice that your ad perform much better in the ad position three and four, you can set your ad for those positions. The position preference option must first be enabled at the campaign level before enabling it at the keyword level. Each keyword in an Ad Group can then have its own position preference setting.
The system will then automatically try to adjust your keyword bid to get your ad in the preferred position. Position preference is not guaranteed and your ad may still appear in other positions. After selecting the ad preference option it might even take the system a few days before delivering the ad accordingly.
It is important to note that your ad will not be served if your ad ranking is not high enough for your preferred position ranking. Your position preference settings also have no effect on your ads on the content networks. The ads average position in your campaign report will give you some indication of what your ad position preference setting should be. It is important to remember that the report might not provide you with an accurate indication if your ad campaign has only been running for a short while. The position preference option can drastically reduce your ad impressions and caution must be taken when using this option.