The 'Trigger' refers to a parameter that you have create in your animator.
Go to Window > Animator
In the top left corner you will see both 'Layers' and 'Parameters'
Click Parameters
Click on the little plus icon
Select Trigger
Give it the name 'Death'
Select the Transition (white arrow) from Any State to your Death animation
In the Inspector-window, under the blue graph, you should see a list of conditions
Click on the plus
Select the Death parameter
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