"Disposed of" relates to the legal term "disposition"--in the context of a will, it means property that the will gives to someone (see
https://en.wikipedia.org/wiki/Testamentary_disposition). Property disposed of by this will means the property that this will bequeaths to someone.
It's possible not all of her property is disposed of by the will. Some of her property may have named beneficiaries, or may be co-owned with someone else; in those cases, it wouldn't be disposed of by the will. For instance, in the US, most retirement accounts have named beneficiaries. If Mrs. X dies, then her retirement account would go straight to the named beneficiary. Similarly, if a married couple owns a house, in most US states if one spouse dies the house goes straight to the surviving spouse automatically. So the will would not dispose of the retirement account or the house; the purpose of a will is to say who gets the property that is NOT covered by some sort of automatic provision like retirement-account beneficiaries or co-owners.
A will may or may not d