Airworthiness Directives

They're probably referring to One-Time or Recurring.

They're pretty self explanatory. A recurring AD requires that an action take place at certain intervals of flight or calendar time, such as "Inspect right wing spar for cracks every 50 hours." or some such.

A one-time is just like it sounds. You take whatever action the AD specifies, and you're done. It might something like, "Replace outer bolt on muffler bearing with improved part, number 1234blahblah."