What does the word trustee mean in a will?

A trustee is an individual ot a trust institution that holds the legal title to property for the benefit of someone else. That person or persons is what is called the beneficial owner. In most US states there are laws pertaining to how much the fees may be for the trustee to charge. By holding the legal title, the trustee has a vested and legal, and moral obligation to see that the property is diversified and that taxes are paid each year.

A trustee is a person to whom control over trust assets (cash, securities or other property) has been placed (by the trust creator) for the benefit of one or more beneficiaries.

Generally speaking a person or persons with powers of attorney may also be involved.

I believe that the word "trustee" has the same meaning regardless of whether it appears in a will or in an written trust agreement meant to take effect while the trust creator is still alive (aka an inter vivos trust).

For example, the testator (the person making his or her will) might want to leave a great deal of money or property to grandchildren who may be still too young to manage it wisely when the testator dies and the grandchildren are in a position to inherit. To delay the grandchildren's actual receipt of the inheritance until they are old enough, the testator may instruct in his will that the money or property is to be held by a named trustee until the grandchildren reach a specified age. The testator should make it clear who is to act as trustee (or alternate trustee, if the first-named trustee cannot serve.)

A trustee has a serious legal responsibility to protect and preserve trust property, and invest it prudently (unless the testator has explicitly forbidden the trustee to invest the funds, which is probably very rare), and never to use the trust principal or income (if any) for his or her own benefit, even if he intends to "borrow" and repay the funds; if he does so, he is breaching his duty. However, a testator or other trust creator may provide in writing that the trustee may be paid his or her fee out of the trust assets, or be reimbursed for any trust-related expenses he or she incurs. Finally, a trustee generally must provide a written accounting of all of the funds coming into and leaving the funds in trust.