Thresholds are defined to fire at predetermined levels (ie, number of free pages) on a segment. When you increase a segment's size (eg, extend the database log => increase the size of the logsegment segment), the previous threshold levels (number of free pages) are maintained.
The warning is simply stating that if any of your previous threshold levels are within 128 pages of the log's new last chance threshold (LCT; automatically redefined when the logsegment segment is increased) then said thresholds will (effectively) be marked as inactive.
If you have thresholds defined on your databases's log (ie, thresholds defined on the logsegment segment) then you'll likely want to re-evaluate said thresholds to determine if you wish to modify said thresholds. For example, if you had previously defined logsegment thresholds to fire at 50/25/10% of available log space, and you want to keep those same percentages for the new/larger log, then you'll need to modify said thresholds based on the larger number of available log pages.
If you don't have any thresholds on your logsegment segment, or you do have thresholds but none of them are within 128 pages of the new LCT, then you can safely ignore this warning.