(To) Grasp (Grab) at Straws

What does the idiom “(To) Grasp (Grab) at Straws” mean?

Definition:

To take desperate actions with little hope of success

For example:

“When the teacher called on her, Jean was grasping at straws—she had no idea how to answer the question.”

Notes:

This is based on the behavior of people who are drowning—they may grasp at straws in the water.