What is the meaning of ˣ?

[x]-fricated release of a plosive (e.g. [tˣ]), sometimes implying an affricate (e.g. ⟨kˣ⟩ for [k͜x]).

[x]-coloring

A weak, fleeting or epenthetic [x]

jäännöslopuke; represents the tendency for a morpheme to cause gemination (rajakahdennus) after it. Always appears at the end of a morpheme.

Source: wiktionary.org