This simple envelope shape raises from 0 volts to its maximum level (typically 5, 8, or perhaps 10 volts) at a speed defined by its Attack parameter, and then immediately falls back to 0 volts at a rate defined by its Decay parameter.
A variation on this is the AHD envelope: After finishing the Attack stage, it holds at the maximum level for a specified amount of time (in contrast to an AR envelope, which holds at the maximum level for as long as the note on gate is high), and then decays back to zero. I have heard there are some envelopes that a hybrid of AHD and AR in that they hold the maximum level for either the defined Hold time or the as long as the incoming gate is high; let me know in the comments if you have a specific example.
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