This means that the page does not react to user input while loading, even though it appears to be fully loaded.
The RAIL model tells us that you must ensure that you process user input within 50ms: this way you should be sure that the visible response occurs within 100ms.
How the Long Tasks are displayed by SeoChecker
SeoChecker will display you in a table like the one below all the long tasks: URL, Start Time and Duration.
How can I optimize Long Tasks?
Firstly, you could split up large scripts and monitor the third-party scripts.
Keep your pages responsive and reduce your work into small parts (not more than 50ms of work), running them in the right place (like off the main thread) and time.