Refers to any assets in your life that do not produce a continual dividend (or benefit) for you. These are not limited to investments; they include relationships, work, memoranda, hobbies - anything that does not produce continual value for you.
Dead assets usually fall into two classes:
Either way, these are things you should consider letting go of or "cashing out" on. But don't simply throw them away: try to derive some value from them first. Show appreciation for the old things you have and how they have served you. Reflect on how the things you've invested in have made you a smarter, wiser, better person.
And then invest your resources - your time, your energy, your money, your focus - onto something more productive.
Note: A person is not a dead asset. A relationship can be a dead asset, especially if it is toxic, but no one in your life is a dead asset, for the simple fact that you don't own anyone. If you're not getting any dividends from a relationship with someone who you think is worth having a relationship with, then consider changing the relationship - not simply getting rid of it.