These structures are diastereomers.
Work Step by Step
If we put a mirror on the left side of the first structure, we will see that the "OH" group is exactly on the same spot as the one in the second structure, but the "Me" isn't. You can also analyze each stereocenter, and you will see that the "OH" groups have opposite configurations, but the 'Me" groups have the same one. Structures with these relations are called diastereomers.