There have been a lot of versions and revisions of D&D over the years. Amongst players of the earlier (or "classic") versions of the game, 'B/X' stands for play based around the Basic Set edited by Tom Moldvay, and the Expert Set edited by "Zeb" Cook, both published in 1981.

The 'B/X' edition of the game came after the "Holmes" boxed set,first published in 1977 and edited by J. Eric Holmes, and before the BECMI (Basic, Expert, Companion, Masters, Immortals) published in 1983 and later (and eventually compiled in the D&D Rules Cyclopedia published in the early '90s). They existed at the same time as Advanced Dungeons & Dragons, which had more, and more complex, rules.

That was a lot of words, to still skip saying Basic | Expert

I appreciated the ancient history lesson ;)

> 'B/X' stands for play based around the Basic Set edited by Tom Moldvay, and the Expert Set edited by "Zeb" Cook, both published in 1981.

Which part was difficult to grok?

A bit of googling suggest this might stand for Basic and Expert dungeons and dragons, after the Basic / Expert boxes the rules came in