$$\frac{d(\ln(f(t))}{dt} = \frac{f'(t)}{f(t)}$$

A Finnish introductory book on differential equations uses the above formula without explanation, simply claiming "it is known". What is this equation and why does it hold true for all `f`? I went through the entire "derivation rules" wikipedia page and could not find it. Searching for the specific formula is hard due to markdown.