What is the meaning of ˚?

released, e.g. ⟨k˚⟩ as opposed to unreleased [].

aspirated, e.g. ⟨k˚⟩ for IPA [] or [].

labialization, e.g. ⟨k˚⟩ (or more properly ⟨kᵒ⟩) for IPA [].

Source: wiktionary.org