What does // mean in Pleco's description of pronunciation?
//

Is pinyin notation for separable verbs and usually more specifically: verb-object phrases.

This is not just Pleco notation. Here's an except from the editorial notes of《四川方言词典》:



And the extracted text:


多音节条目按词连写。无论词或词组，凡中间可插入其他成分的，动宾结构用“=”隔开，动补结构用“//”隔开。如“胎包袱 tai1 = bao1 fu2”、“捡顺 jian3//sun4”.


Here's an example of an entry that uses “//” from inside the same book:



And its extracted text:

归顺
gui1//sun4
（动）收拾妥贴：房间里的床铺已叠整齐，桌上的杂物书籍全～，摆好。(现84·12·51)

《四川方言词典》uses &quot;//&quot; for verb-compliment constructions and &quot;=&quot; for V.O. constructions but these usages may have merged later.

I believe this notation has already been removed from Pleco though as it caused too much confusion for learners unfamiliar with this usage.