First of all, I am sorry for your doubtless painful situation and impending loss.
Most toddlers do not have a firm grasp on the 'facts of life' yet. They are constantly making up theories, based on their experiences and observations, and then rejecting them again when new observations don't match their theory.
So the basic trajectory through life - being born, growing 'up' i.e. going through the developmental stages to adulthood, then growing 'older' whilst not fundamentally changing, then growing 'old' i.e. body functions decreasing, then dying - is not yet understood. For example many children at some stage develop a theory that they used to be a grown up before they became a baby. Both my kids used to ask things like 'When I was a grown up and you were a baby, did I used to do X?' and similar.
And so death cannot easily be explained in these terms of a trajectory through life that we as grown-ups take for granted.
Similarly, toddlers struggle with the very abstract concept of 'absence' of something, so 'absence of life', 'ceasing to exist' won't be something most toddlers are developmentally capable of grasping.
Also, though we don't tend to like to think about it, our own concepts of death can be quite unclear and confused, and it is hard to explain to a little person something that we don't properly understand ourselves.
So I would advise you to first consider for yourself what death means to you. This may depend on your own attitudes e.g. to religion. Is death solely about the body, but the soul 'lives on'? Depending on that, you can then try to formulate an explanation that your child is able to grasp.
I would also advise to actually focus more on 'grief' than on 'death' itself. Your child does not need to have a definitive rational understanding of what death is. But they do need help in understanding, naming, and appropriately expressing their own feelings, and in understanding the grief their loved ones (e.g. you) will be feeling (and are feeling already, no doubt).
When DS was two, we talked a lot about how death is 'sad but not bad'. Acknowledging that whilst death is a natural, normal thing to happen, and needs to happen to make room for new life, and for the person dying it may be a relief from suffering; it can still make us very sad. The person who has died is no longer in our life and we miss them.
In terms of books, perhaps you'd like to look at 'The Paper Dolls' - it is a lovely book that includes the point that things and people we 'lose' don't cease to exist, but rather 'fly straight into our memory' and continue to 'be' there.
Also 'Goodbye Mog' especially if your child knows the Mog books already. Mog's 'ghost' lingers on for a little bit after dying, and observes the family being sad, and eventually getting a new kitten who does everything 'wrong' until Mog gently helps the kitten settle in, upon which Mog's soul is content and can 'go' and the family can move on and keep living and find some happiness again, whilst always remembering Mog.
And finally 'Badger's parting gifts' - this really focuses on the grieving process. Badger 'gifts' each of his friends a positive memory of himself to hang on to and share with each other and so the bereaved friends help each other in grieving.
Wishing you lots of strength for the coming times. Having children can really heighten all aspects of the death of a loved one, but can also be a source of comfort and something to hold on to and to help you keep on keeping on.