Olimar has only 30 game days to find all the missing parts of his ship. The planet is much too dangerous to remain on the surface at night, so he has only 12 game hours each day; this amounts to about a quarter an hour in real time. At the start of a day, Olimar can draw Pikmin from their onions, then direct them to return parts of his spaceship, break down walls, build bridges, gather food to make more Pikmin seeds, etc. Meanwhile Olimar can pluck Pikmin sprouts to grow their numbers and replace those lost in battle. When the end of the day is near, Olimar must gather any straggling Pikmin and bring them to the ship area, otherwise they will be lost to the predators that roam the planet at night.
Since you have a limited time to collect all the ship parts, the key to successfully completing the game is multitasking. For example if you've just started a group of Pikmin building a bridge, something they can do without supervision, leave them to it and take another group to do something else in the meanwhile. Planning each day to create opportunities for multitasking makes the difference between completing the game with the ship fully repaired and leaving Olimar stranded.