被 is for passive. 所 creates a relative clause, which is usually not translated in full, because it would look very clumsy. Instead, 被 and 所 used together are usually translated simply as a passive construction.
The basic answer has already been provided by songyuanyao: bèi 被 and suǒ 所 form a passive construction. I will add a source and try to explain.
While the Chinese language has changed a lot over time, certain aspects of grammar and the usage and function of certain structures and words has remained the same in principle.
Writing about passive constructions with 爲 (which in modern Chinese is replaced bei 被 or 將), Edwin G. Pulleyblank points out in Outline of Classical Chinese Grammar (p. 37):
In later Literary Chinese, from about the beginning of the Hàn
dynasty, this construction takes on a new form, in which suǒ 所 is
inserted in front of the embedded verb. That is, wéi sān jūn
huó[sic] 爲 三 軍 獲 [meaning: being captured by the three armies] would
become wéi sān jūn sǔo huó 爲 三 軍 所 獲. As we shall see, suǒ 所 is
the regular substitute for the object of a verb in a relative
clause when this is coreferent with the head of the clause. The noun
after wéi 爲 continues to be its indirect object, not the subject of
the relative clause, since it is never followed by zhī 之 as a mark
of nominalization and since it takes the object pronoun zhī 之 rather
than the possessive pronoun qí 其 as its pronoun substitute.
So, in passive constructions with bèi 被, a suǒ 所 can be used.
What does 所 do?
It replaces the object of a verb and alters its position in the sentence to stand in front of it.
In your case the verb is 掩蓋.
The object of 掩蓋 would be gold. In other words, 所 creates a relative clause that could be translated as “that which is covered”.
Remember that the subject of your main sentence is still 他, standing for 金子, the piece of gold.
As Pulleyblank explains, the noun 髒東西 after the passive marker is not the subject of the relative clause, but the indirect object.
That means, we cannot directly translate to “that which the dirt covered”.
Then in the next section (p. 37f):
In its new form we must therefore construe our sample sentence as ‘You
will be for the Three Armies what [the Three Armies] capture.’
With both wéi 爲 and wéi suǒ 爲 所 the agency may be left unexpressed.
Therefore, we’d first analyse [他/金子]被脏东西所掩盖 as “[It/The gold] will be for the dirt, what the dirt covers” and then transform it into an English sentence which more elegantly expresses that meaning: “Eventually, the poor piece of gold ended up being covered in dirt... Again!”