Instagram allows you to enter a caption each time you upload a photo from your device, but the description applies only to that picture specifically -- it doesn't apply to your entire account. If you want to add a bio to your account, edit your profile from your computer or mobile device.
From Your Browser
Log in to Instagram and select your username from the top of the page. Choose "Edit Profile" from the drop-down menu, enter your description into the "Bio" field and then click "Submit" to apply your changes.
From the App
Launch Instagram in iOS, Android or Windows Phone OS, and then touch the profile icon (it looks like an ID card) on the bottom right. Touch "Edit Your Profile" and then select the "Bio" field. Enter your description using the on-screen keyboard, then touch "Save" for Android, "Done" for iPhone, or press the check mark for Windows Phone.