The gemara is discussing a mixture which has avodah zarah (asheira tree) in it. Tosfos quotes Rabeinu Tam who notes the correct girsa is "nisarvu b'acheiros", it was mixed with others, and not "acheiros b'acheiros", that mixture was further mixed into another mixture. What difference does that gursa make? Rabeinu Tam (Tos spells this out elsewhere) wants to avoid creating a "sefeik sefeika", a double-doubt, which is normally mutar.
Tosfos elsewhere cites the Rivam who adopts the girsa that R"T rejects. The Rivam writes that "acheiros b'acheiros" is like a double-rov. Imagine a piece of asheira that is 1/100 of a mixture. If 60/100 of that mixture fall into another mixture, since 60/100 is a rov, it is as if the issur itself fell into the second mixture.
The Rivam sounds reasonable - why would Rabeinu Tam not agree with that explanation? (To be continued... as usual, you get a day or two to think about it ; )