A convention card is a list of bidding conventions. A partnership uses a named convention card, allowing the partners and opponents to correctly interpret the auction.
On OKbridge, pick-up partnerships are quite common. To avoid long discussions regarding systems and methods, many of these partnerships use the OKbridge version of the Standard American Yellow Card (SAYC). You will often see a quote similar to "sayc ok pard?" Your partner is asking you if you will play this convention card.
OKbridge includes a few popular standardized convention cards that partnerships may use, including SAYC, 2/1, Goren, ACOL, and Precision. You are not restricted to these convention cards, you can make up any convention card that you like.
Whatever convention card your partnership agrees to use must also be known to your opponents. When your partnership uses a standardized convention card, you can simply name the card and your opponents can see it on OKbridge. When your partnership uses a customized convention card, you must publish the card before the game begins so your opponents can see it.
You can access the dialog to view standardized convention cards, and create and publish your own convention cards by clicking the CC tab in the Information pane.