don't understand by the phrases 'third second'

[Sanosuke Sagara]

My question is in the attachment.Thanks for anyone that gives help !

[jamesrc]

They want to know how far the object goes between time t = 2 and time t = 3. (0-1 is the 1st second, 1-2 is the second second, and 2-3 is the third second).