Analysis of Sinaiticus (yes, א is the symbol) has led to the conclusion that there are three general periods of additional editing. So the symbols represent information about these periods. Per the NA28 Introductory material on the critical apparatus:
- א by itself means the only reading present.
- א* Is a notation for the original reading when a later revision exists.
- א with the 1 superscript is a revision from 4th-6th century.
- א with the 2 superscript is a revision circa 7th century
- א with the 3 superscript is revision circa 12th century.
The superscripts follow א in the NA28 apparatus, but appear to proceed it in the NET notes.
You can see the correction on the manuscript here (at the top of the page, above the second column, noted with a what looks like a % sign which is the symbol used to locate where the correction should be in the text below).
For a more detailed exchange about the textual issues related to Mat 17:21 with respect to Mark 9:29, see this question on BH.SE.