How Advertising Works
From Legacy Roblox Wiki
Creating an ad
To create an ad, go to one of your place's pages' and press the link "Advertise this place" underneath your avatar.
This will bring you to a page where you can get the ad templates, and upload ads.
Using an ad
Once you have created and uploaded an ad, you can now display it. Go to you profile and press "add inventory". This brings you to a page where you can run you adds on the site. To start running an ad, press "Run ad" on the right side of the pane. You have to bid money to get you add put up over other people's ads.
Ad statistics
Impressions: How many times your ad has been shown on a page so far.
Clicks: How many time people have clicked your ad.
CTR: What percentage of showings resulted in people clicking on the ad.
Bid: How much you bid