Time to Interactive (TTI) marks when the page is reliably ready to respond to input: the page has displayed useful content (FCP fired), event handlers are registered for visible elements, and the main thread is free of long tasks for at least 5 seconds. The user can click, type, or tap and the response will be quick.
TTI is a Lighthouse lab metric only -- it's not a Core Web Vital and Google has deprecated it in favour of INP for measuring interactivity in the field. As of Lighthouse 10, TTI is no longer included in the performance score, though the audit still surfaces the value for debugging.
If your page has poor TTI, the cause is almost always too much JavaScript executing on the main thread. The fix is the same as fixing INP and TBT: code-split, defer non-critical scripts, and break up long tasks.