Why has zuò//láo // in between? I am going through the word 坐牢 using the Pleco dictionary. Generally they will not keep // in between two characters. But I can see // for this.
zuò//láo
What does // mean? Can zuò and láo be used with some sentence in between? Or is there any other meaning?
The example given here is some sentence in between.
他坐过三年牢。
Tā zuòguò sān nián láo.
He was in prison for three years.
The // is the marker used by Pleco to indicate that the word is in a special class of intransitive verbs called separable verbs.
Other references call these V.O. (Verb Object) constructions. ABC dictionary gives a useful TL;DR summary:

V.O. (Verb-Object Construction, Dòng-Bīn Jiégòu 动宾结构). Many English
verbs get translated into natural Chinese as a verb plus an object
noun, e.g. chīfàn for 'eat', shuōhuà for 'speak', etc. It is important
for two reasons to know what is merely a verb in Chinese and what is
actually a verb-object construction. First, verb-object constructions
can never take a second object, i.e. chīfàn can never be followed
directly by something else to be eaten. Second, a verb and its object
can be separated from one another, thus allowing (i) aspect particles
to be placed directly after the verb, e.g. chīle fàn 'after finishing
eating'; (ii) modification of the object, e.g. chī Zhōngguófàn 'eat
Chinese food'; and (iii) quantification of the noun, e.g. chīle sān
wǎn fàn 'ate three bowls of rice'.

The notion of separable verbs can be a very useful thing to be aware of as a student. This introduction and this list of separable verbs from allsetlearning.com may be helpful references on this topic.