The usage bar for my iPad on iTunes window on my PC tells me that I am devoting 6.1 gb to "other", apps take up about 3.7 gb. What is "other"?

The usage bar for my 16 GB iPad on the iTunes window on my PC tells me that I am devoting 6.1 GB to "other", apps take up about 3.7 GB. What is "other" and how can I reduce the space I am using here. With only 4.1 GB left in storage space I may not now be able to download a movie from iTunes.

iPad

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