What did Sheikh Al-Uthaymeen mean by his statement that Ahad (single-chain) narrations convey presumption and action, and perhaps even certainty if supported by corroborating evidence?

Ahad (singular) Hadith (definition) is that which does not reach the level of Mutawatir (mass-transmitted), and it includes the famous (Mashhur), the عزيز (Aziz), and the strange (Gharib). The weak (Da'if) Hadith is excluded because it does not convey what the sound (Sahih (definition)) Hadith conveys. The sound (Sahih) or good (Hasan (definition)) Ahad Hadith conveys probability (al-ẓann), which is the preponderance of the correctness of its attribution to the one from whom it was narrated, unless it is accompanied by circumstantial evidence (qara’in) that elevate it above possibilities of error or forgetfulness. It is obligatory to act upon its dictates in rulings and to believe in its content, whether in matters of creed, worship, or transactions.