Auto time adjuster is a plugin delay compensation (PDC) plugin, originally designed for use in Pro Tools LE 7 & 8 (RTAS), as they lacked built-in PDC. ATA continues to be a useful tool with newer versions of Pro Tools (9+/AAX) that include PDC, as users can benefit from the large latency limit (50,000 samples), and the increased accuracy of measuring plugin latency directly (including outboard), rather than relying on the values reported by plugins.
ATA is inserted on each track in your mix. The actual latency of all plugin and outboard inserts is then measured using a short burst of audio, or 'ping' triggered from the ATA inserted on the master fader. ATA then calculates and sets appropriate delays for each ATA on each track or bus, bringing all tracks into alignment and providing the tightest timing possible.