How Do People Find Your Site?

•by keyword search on search engines

SE traffic and directory listings are partially profitable for AdSense participants. They mix targeted traffic and bystanders, thus, although not all traffic sent to your page will be “productive”, there'll still be some visitors interested to click on your ads.

•by coming across links on pages/sites that are related to your theme. These are likely to bring mostly targeted traffic.

So, make your site findable by:
•keyword optimization, good Meta description tag, title tag, header tags, optimized ALT tags applied to images (on-page SEO techniques)

•link building (off-page SEO technique) — you benefit doubly from this: more people are likely to find you more easily on other sites and you get higher PR (Google™'s PR algorithm takes into account a site's link popularity, on condition they are relevant links) obtaining thus better SERP positioning.