In context, Helen Keller was referring to her college days. Before college, she had a more leisurely life in which she could spend time alone with her thoughts, enjoying the outdoors, reading her favorite books, or doing whatever she pleased. However, in college, there was no time for this. Now, she had to spend every moment learning new things in school.
Helen admitted, "I suppose I ought to find some comfort in the thought that I am laying up treasures for future enjoyment..." She knew that she should be grateful for college because it was giving her something to use later on in her life. College was much more useful to her at that time than daydreaming or wandering in the woods, and she realized this. She continued, "...but I am improvident enough to prefer present joy to hoarding riches against a rainy day.” Being "improvident" means you are being careless about your future, and not preparing ahead. So she honestly admitted that she would much rather enjoy her life in the "present joy" by being lost in her thoughts or books than to bear the drudgery of studying--even though studying is obviously the wisest choice for her future career, just as "hoarding riches" is wise to prepare for a potential future calamity, or "rainy day."
We do know that Helen went on to be a very educated and successful woman. But in context, her quote was lamenting the strict discipline of college in contrast with her love of having free time in which she could do as she pleased.