MicroPro Sienna Brown wood

It's pressure treated wood. It shouldn't be used indoors because if there's a fire, it releases lots of nasty chemicals compared to plain whitewood. Also shouldn't be sanded or cut without wearing a mask because you'll be breathing in that stuff. Keep in mind that treatments will cause corrosion with non-galvanized metals. I've read of people placing metal tools down on PT wood and when they pick it up again a couple of days later, they already start to see some corrosion. MicroPro Sienna is supposed to not corrode as much compared to other brands, but still...

Current treatments aren't as bad as the older stuff, but still not very "healthy". I think Bluwood is the other pressure treatment allowed indoors.

All that being said, if you're just building a workbench, you very likely won't suffer any harm from it, whether you leave it in your garage or inside your house, but don't frame a basement with it. If I had a lot of extras, there are lots of exterior projects you could use for it: benches, side tables, small gazebo for your BBQ, hidden storage for garbage bins, etc.