A hue, some white, some black
A mid blue like #3A7BD5 is hwb(214.84 22.7% 16.5%). The first number is the hue, the same angle round the colour wheel that HSL and HSB use, so 0 is red, 120 is green and 240 is blue.
Whiteness is how much white is mixed in, blackness is how much black. hwb(0 0% 0%) is pure red. Add white and it heads towards pink, add black and it heads towards maroon, add both and it goes greyish. That's it - two knobs that do exactly what they say on the tin.
Once whiteness and blackness add up to 100% or more there's no room left for the hue, and you get a flat grey. hwb(0 50% 50%) and hwb(200 50% 50%) are the same middle grey.
Where you can use it
hwb() is proper CSS and works in every current major browser, so you can drop it straight into a stylesheet. Unlike rgb() and hsl() it never had a comma version, so it's always spaces, with a slash before any alpha: hwb(215 23% 17% / 0.5).
Design tools mostly don't show it, and you won't find many brand guides written in it. It's more of a writing-by-hand format, handy when you're tweeking a colour in code and want to know what the numbers will do before you save.
How it relates to HSB
HWB was designed by Alvy Ray Smith, the same person behind HSV, as an easier way to think about the same thing. The maths is a straight swap: whiteness is one minus saturation, times brightness, and blackness is one minus brightness.
So every point in a design tool's HSB square has an HWB address too. Whiteness grows as you move towards the left edge, blackness as you move towards the bottom.
What it won't do for you
Like HSL and HSB, HWB is built straight on top of sRGB, so equal steps don't look equal. A yellow and a blue with the same whiteness and blackness still look miles apart in brightness. For palettes where that matters, have a look at OKLCH.