Bodhisattvas play a smaller role in Theravada Buddhism than they do in Mahayana Buddhism. Every person who becomes a Buddha had previous lives. The last few of those lives they were very very close to attaining Buddhahood, but not quite there yet.
Hence Gautama Buddha was, technically, a Bodhisattva (Pali - Bodhisatta) from the time he was born until his enlightenment at age 35. He was also a Bodhisattva in his last few previous lives. His immediately previous life was spent in the Tusita heaven.
The next Buddha to appear on earth, the Maitreya Buddha is currently a Bodhisattva in the Tusita heaven awaiting his final rebirth on earth.
Other persons on earth may also be Bodhisattvas, but they will be further from attaining Buddhahood than Maitreya.
Since Bodhisattvas are not yet fully enlightened, they still have some areas of ignorance. The closer they are to their final rebirth the less ignorance they have.
Becoming a fully enlightened Buddha is difficult. For most people it is easier to become an arhat, given that the Buddhist religion is currently being taught. It will eventually die out on earth, and be rediscovered by the Maitreya Buddha. In the interim, being an arhat will not be possible as the teaching is no longer available. That is why being a Buddha is more difficult, the teaching has to be rediscovered rather than learned.
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