It means it is your forever, unless it is taken back, the reasons for which would be described elsewhere in the contract. The language may not be lay language, but neither was the ream of paper it took to buy my house, but people still do that every day.
Digging into this a little deeper, even "irrevocable" isn't as clear-cut as one might expect.
> An “irrevocable” license, on the other hand, cannot be terminated, although there is some divergence in authority regarding whether this means that the license cannot be terminated for any reason or only that the license cannot be terminated for convenience, but still may be terminated for breach.
To my layman's ear the two terms sound almost contradictory.