Inventory is slots to put ads. AOL had the eyeballs, Google had the platform to turn eyeballs into money, better than AOL was doing by itself. Google provided a guaranteed rate (cpm? cpc?) so that AOL would have confidence it wasn't bait and switch.
See also the Yahoo/Bing deal which didn't work out as well. Microsoft didn't end up actually hitting the targets, and convinced Yahoo to take less; and Yahoo also didn't reduce employee count anywhere near plan on searchy/advertising stuff, so they missed targets on revenue and cost and user experience.
It was guaranteed revenue IIUC, something like "If you do not make at least $150M from this deal, Google will pay you the difference." That makes the deal a no-brainer for AOL, but puts Google on the hook for any shortfall, which could have potentially ended up bankrupting the company.
I couldn't find a great description of the deal terms. I did find mention that at least some of the guarantee needed to be paid up front. I suspect the guaranteed payment was contingent on some level of traffic though -- if AOL had turned off the placement, or lost a huge amount of users, then they probably couldn't keep the money.
See also the Yahoo/Bing deal which didn't work out as well. Microsoft didn't end up actually hitting the targets, and convinced Yahoo to take less; and Yahoo also didn't reduce employee count anywhere near plan on searchy/advertising stuff, so they missed targets on revenue and cost and user experience.