Warning: Spoilers follow
She is the mother of Princess Leia and Luke Skywalker, and democratically-elected Queen of Naboo, and later represents the planet as a Senator in the Galactic Senate. Amidala plays an important role in the last years of the Old Republic.
In The Phantom Menace, she fights hard through political and violent means (of which she participates in personally) to rid her planet from the blockade of the Trade Federation. Aged in her early teens in this movie, she forms a close friendship with the very young (nine-year-old) Anakin Skywalker.
In Attack of the Clones, set ten years later, she is a Senator in the Galactic Republic's Senate after her two terms as Queen expired, leading a faction opposed to the establishment of an army to quell a separatist movement (surprisingly, given her career change, she retains an elaborate fashion sense and a constantly-changing wardrobe). After an assassination attempt on her life, Anakin, now an apprentice Jedi, is assigned to protect her. The two fall in love, though both of them are duty-bound to deny it. In the process of rescuing Obi-Wan Kenobi, they become directly involved in the first massed battle of the Clone Wars, where Amidala again shows herself to be rather more handy in combat than might be expected of a politician. The movie ends with Anakin and Amidala marrying in a secret ceremony attended only by R2-D2 and C-3PO.