Pretty common terminology for rolling another die and adding it to the roll value. In this case it'd be you roll another die or the same die again and add it to the total when you roll a 5 or 6.
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Basically they explode once, it's actually the first sentence of the actual rules: 1d6 system. On a roll of 6, roll the die a second time and add it to the total.
However the most recent version of the game (waiting for reviewing to end) does change the language on Energetic to not use the term exploding, and better resemble other rule descriptions.