What does "she planted her feet in first position, standing erect like a wooden toy soldier" mean?

This is a simile.  First position is a stance in ballet where the feet are placed in a position that point directly to the left and right.  For instance, if a person’s nose was considered to be pointing to 12 o’clock and the back of person’s head was considered to be pointing to 6 o’clock, the left shoulder would be considered to be point to 9 o’clock, and the right shoulder would be considered to be pointing to 3 o’clock.  First position in ballet is where a person puts the heels of their feet together and points the left foot to 9 o’clock and the right foot to 3 o’clock while standing up straight.  “Like a wooden toy soldier” means that her posture was probably close to perfect, so much so that her stance could be equated to that of a soldier standing at attention.  And, she probably was standing so still that she resembled something as immobile as a statue or toy, or in this case, a toy soldier.

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