Think of it like a wall made of trees that have been chopped down and arranged to point outwards, making it hard for enemies to get through. Sometimes the branches are sharpened or have barbed wire to make it even nastier.
The soldiers built an abattis around their camp to protect themselves from attack
The defenders used an abattis to slow down the advancing army
Imagine a bunch of Christmas trees laid down with the pointy ends facing the bad guys - that's an abattis
