How it works
2-String Kite is a technique that involves 2 conjugate pairs for a single digit, with one pair located in a row, and the other in a column. These are the strings. The pairs are linked in one box, which represents the kite.
The candidate for the digit can be removed from the cell that can see both ends of the strings.
Here, the kite is in box 1, the strings in row 1 and column 1 respectively. Either the cells marked with blue, or the cells marked with green can have the digit. This shows that the red cell can never become the digit, so any candidates in it can be removed.
Examples
In this example we have a 2-String Kite for the digit 2, with the kite located in box 9, and the strings in column 7 and row 8. The rules tell us that if 2 is placed in r8c8, then it must also be placed in r1c7. If it is placed in r9c7, then it must also be placed in r8c4. As r1c4 can see both ends, the candidate for 2 can be eliminated.
This example additionally shows that neither r8c8 nor r9c7 have to contain the digit 2 as it might be placed in r7c9 or r9c9. If this is the case then both r1c7 and r8c4 must contain the digit, and the candidate in r1c4 still have to be eliminated.