A hex code is three numbers in base 16
#0E6B54 is not one number, it is three: 0E, 6B and 54, each a base-16 value from 00 to FF describing how much red, green and blue to emit. FF is 255, the maximum for a channel in 8-bit colour. Once you see the code as three pairs, the conversion is simply reading each pair in base 16.
Three-digit shorthand
CSS allows a three-digit form where each digit is doubled: #0B5 expands to #00BB55. It is a convenience for round values only — there is no three-digit way to write #0E6B54, because 0E cannot be produced by doubling a single digit. The converter accepts both forms.
HSL is easier to reason about
RGB is how a screen works, not how people think about colour. HSL restates the same colour as hue (an angle on the colour wheel), saturation and lightness, which makes it far easier to produce a deliberate variation: keep the hue, drop the lightness, and you have a darker version of the same colour rather than an unrelated one. Both are shown for that reason.
Why contrast appears here
If you are converting a brand colour, the next question is almost always whether text is legible on it. WCAG 2.1 defines contrast as a ratio between relative luminances, and asks for at least 4.5:1 for body text or 3:1 for large text. The contrast rows show the ratio against black and white with the verdict, so the check happens without leaving the page. Note that luminance is not brightness as your eye judges it — green contributes far more than blue, which is why the coefficients differ.