The word boat on a card -example- caledonians or icon on the gameboard show or tell nothing as what to do, rounds 4,,6,. yes on round 5 on card

The note on the nations card indicates that during that turn the tokens of that nation can use Boat Movement - this is explained on Page 9 of the rulebook on the right hand column.

I have seen boat movement on page 9. What im trying to say is there is no units or invasion, no sea zone during turn 4 The caledonians don't start holding untill turn 5.thank you

The Caledonians are on board from the start of the game so while they never get an invasion step, they build their population and then can expand so there are spare armies to advance - usually into the Picts. Boats on turn 4 5 and 6 allows them to potentially go behind the Picts - its usually used to send armies from one of their territories like Orkney or Caithness over to Mar or to threaten to do so, forcing the Picts to defend in depth especially if the Brigantes decide to give the Picts a beating.