Real-time bidding, or RTB, is the auction mechanism that decides which ad appears the instant a page loads or an app opens — a live bidding process that runs and resolves in the milliseconds between a request and the page rendering. Instead of buying a fixed block of ad space in advance, advertisers submit standing rules that let their bid be recalculated for each individual opportunity based on who the viewer is, what device they're on, and how likely that specific impression is to convert. For product feed advertisers, RTB is the layer underneath both Google Shopping and social catalog ads that determines, auction by auction, which products actually get shown and at what cost.

Why it matters for feed management and ad tech

Because RTB reprices every single impression independently, a product's eligibility and competitiveness in that auction depends entirely on what the feed says about it at that moment — price, availability, and category all feed directly into whether an item even qualifies to compete. A product marked out of stock or missing a required identifier is excluded from the auction before bidding even starts, no matter how well bid management has tuned its target bid. This is why feed freshness matters more under RTB than it did under older, slower ad-buying models: an auction happening in real time has no tolerance for stale data, and a feed that updates once a day can lose an entire day's worth of winnable auctions to a competitor with fresher stock information.

How it works

Each time a user's browser or app requests an ad slot, an exchange sends the opportunity to eligible advertisers, who each return a bid generated on the fly by their bid strategies engine. The exchange scores incoming bids alongside relevance and predicted click-through rate, then the highest-scoring bid wins and its ad is rendered — all before the page finishes loading. For shopping ads specifically, this process also filters on structured feed data: an item's category, price competitiveness, and shipping terms all factor into whether it wins that particular auction against a similar product from another retailer. Over many auctions, the share of eligible opportunities an advertiser actually wins becomes a trackable metric known as impression share, which is often the clearest signal that either bids or feed quality need adjustment.

Example

<item>
  <g:id>SKU-77310</g:id>
  <title>Merino Wool Hiking Socks - Size 9-11</title>
  <g:price>18.50 USD</g:price>
  <g:availability>in stock</g:availability>
  <g:google_product_category>Apparel & Accessories > Clothing > Socks</g:google_product_category>
  <g:condition>new</g:condition>
</item>

None of these fields set a bid directly, but every one of them determines whether this item is even allowed to enter a real-time auction in the first place — an availability value of out of stock or a missing google_product_category removes the product from contention before price ever gets compared.

Related Concepts

RTB is the mechanical layer that both bid management and bid strategies ultimately execute through, since neither a manually adjusted bid nor an automated target-ROAS rule means anything until it's resolved inside a live auction. Advertisers monitoring how well their catalog performs under RTB typically watch impression share as the leading indicator, since a shrinking share often points to an auction-eligibility problem in the feed before it shows up anywhere else.