In many application essays you may need or want to talk about multiple experiences. (many law school personal statements and AMCAS essays fall into this group. Some MBA essays, such as HBS 2, also ask about more than one experience.) So if you are discussing disparate topics, like overcoming personal hardship, exposure to your field, and a distinctive hobby of yours, how do you tie them together into a coherent essay?
You look for the common thread in those experiences.
Sometimes it will stare you in the face, as it would for the medical school applicant who personally dealt with serious illness, acquired an interest in medicine, pursued it in college through research, clinical volunteer work and leading a group of students volunteering at a local clinic.
Depending on what you are writing about, you may find it quite difficult to ferret out a theme (see Change, a sample law school essay on Accepted.com). In such cases, the common thread may be something like a character trait, irony, serendipity, or a diversity of interests, but regardless of the type of thread, every essay needs one to hold it together.
For more on weaving your essay with a common thread or theme, please see the following tips and articles: