I've got a Hoya ND4, the nd standing for natural density. This is used for situations where you want a slow shutter speed or wide aperture, but there's bright-ish daylight. It's nuetral - not filtering any particular wavelengths. It just slows things down a bit. Handy for daylight motion blur or when you want a narrow dof on a sunny day.
The rest of the filters are B+W - a brand name I've just learned. The B+W KR 1.5 is a skylight for colour. I've used one of these plenty in the past, and would leave it on the camera all the time with a colour film during the day.
The B+W KB6 is a cooling filter, which, apparently, you'd use if you were shooting colour film at sunrise or sunset and wanted to make the light less red. Can't think I'd use it that much, but you never know.
The third of the B+W filters has no descriptor letter and number on it. The eBay seller described it as "warming", so perhaps it's an 81A or B. That takes the blueness out, too, like a skylight.
Researching those lenses, you get lots of US sites selling them, which shows I've gotten a bargain - I think I paid about £15 for the lot. I've got red, green and yellow filters for black and white (as opposed to B+W!), and I think I've got a UV filter (when we unpack). Which means I've got all the filters I could ever need at 40.5mm. The hilarious kicker is, Leica appear to use a slightly different filter size, so when I go Leica, my current comprehensive filter collection won't work on the Leica lenses.