I'm new to the terminology, so let's say the 9x9 boxes are represented like so: ABC DEF GHI
One 8 is missing from each of the three middle boxes D,E,F. In column 2 (Box D), an 8 must be in row 4 or 5. In column 6 (Box E), an 8 must be in row 4 or 5. In column 7 (Box F), an 8 must be in row 6 (by elimination of 4 and 5).
See how the two blanks in box C are vertically aligned? Without knowing what order they're in, you know one is a 2 and the other is a 6 (the two remaining digits for that box).
If you assumed they were filled in and took up the only available 2 and 6 for their column, you might uncover a 2 or a 6 in one of the other two columns in box F.
It could be helpful to read the General/ Program section of this forum - there's a sticky called 'Basic Terms' which explains how we can all sing from the same song sheet - though of course some of us will always have better voices than others!