Focus is similar to a magnifier, too far or too close your vision will be blurry.
When using your laser machine you want to keep two things in mind.
1: When you engrave you want the focus on top of your material
2: When you cut, you want the focus in the center of material thickness
When you have learned the machine, you can use out of focus as a effect to make your engravings but this takes a lot of testing and training to identify what creates the effect you want. Stay with the “in focus” solution until you learned how it works!
Here is a picture showing the focal point, often at 50.8mm from lens underside when cutting.
If your focus is too high or too low your cut will have slanted sides, and it might demand much more power to cut the material.
If you get slanted edges when cutting, take a look at your focus and change the distance to the material to sort it out.