Question about “walkers”

When people refer to walkers, what are they talking about? I’ve heard they’re unsafe, but what kind are they referring to?

Do they mean something like this:

Or is it something like this:

Or are both of them unsafe? My daughter pushes around her activity table, her toy basket, our small ottoman, etc. basically anything that is at a height she can stand up with, and not too heavy for her to push, but I was thinking about buying her a “walker” like the second picture, so she can walk behind it and practice her skills more easily without scraping my furniture and her other toys on my hardwood floors, but I don’t want to do it if someone thinks it’ll stunt her development. I just don’t understand how this would be any different from her pushing other things around without wheels. Please help me to understand what “walker” means.