Looks like spalling due to water seeping upwards from the end of the garage slab on the outside and then blowing off the surface when the sun got onto it. Also looks like this happened before the outside ramp strip was laid so it may not be a problem now.
To fix it you need to vacuum it to remove dust and use a patching product to fill up the holes, preferably epoxy based. Then the best way to smooth it is to hire a handheld diamond grinder with dust head and vacuum.
Another way is to buy some thinset and trowell over, do it a second time after the first has cured because the first application will sink in the holes a bit. It may even require a third, but be careful not to leave anything on the slab or you will have to grind it off and also do not wait too long before the second or third application, you do not want it to cure too long (days) before the next application, should be instructions with it.