What a relief! Wikipedia says that Tortoise's music defies easy categorization. I agree with those "experts". I don't know exactly what it is. It is often soulful or sad, occasionally raucous and joyous, almost always instrumental. Listening to this album takes you through a variety of moods. It would be appropriate piped(!) through your headphones as you take an afternoon nap in your La-Z-Boy.
Thumbs up.
About the album cover image: "Millions Now Living Will Never Die - Tortoise" by Source. Licensed under Fair use via Wikipedia.