To entrench is to secure something firmly. Entrench a tent pole in the ground so your tent doesn't fly away, or entrench yourself at your new job so you don’t get fired.
Dig a little and see that entrench is from en meaning “in” and trench is from the Old French word trenche for "ditch,” just like the kind soldiers hide in. You don’t have to be a soldier to entrench, though — anytime a person or thing is securely inside something, it’s entrenched. If you entrench a swing set in your yard, it's solid. If a government entrenches a spy in another country, that spy blends right in.