Flutter means to move back and forth rapidly. Flags flutter in the wind. Leaves flutter to the ground. Flutter also exists as a noun—you might feel a flutter in your heart when you're excited.
To remember flutter, think of the children's rhyme, "See the butterfly flutter by." Flutter is often paired with "flit," to describe the way that birds and insects fly––they flit and flutter. It can also be used to describe indecision—someone who can't make up their mind might flutter between two choices.