This is the 3486th most frequent Japanese word.
確
"確" means "certain," "sure," or "firm."
In this sentence, 確か (tashika) is used as an adjective meaning 'accurate' or 'reliable'.
その情報が確かかどうか、確認したいですね。
I want to verify whether that information is accurate.
Here, 確実 (kakujitsu) is used as a noun meaning 'certainty' or 'reliability', often indicating a high probability.
彼は確実にその仕事を終えます。
He will definitely finish that job.
In this example, 確信 (kakushin) is used as a noun meaning 'belief' or 'conviction', signifying a strong assurance about something.