Caps are daily, monthly, or per-partner limits on how many conversions can be run for the offer or how much the advertiser is willing to pay for traffic. This is set by the network administrator and is displayed in the "Offer Details" screen. If an offer's monthly cap is set to 10,000 conversions and you convert 10,001, the offer will only pay out for the first 10,000 conversions.
The network may also set a payout cap for an offer. If the offer payout cap is set to $1,500 and you've run $1,750 conversions, the network will only pay the first $1,500.
To receive notifications when the cap is approaching, enable notifications by navigating to the "Settings" menu under "My Account." Once there, you can enable the notifications you'd like.