The Apache 2 license is considered as permissive. You can include Apache 2 license code in your commercial application and you can distribute/sell your software under an other license (commercial) without the need to redistribute the source code.
Your only requirement is to add a notice text for informational purpose only. For example, include somewhere (for example in the About menu): This software includes parts of "KLNoteViewController" under Apache 2 license.
Keep also the Copyright notice in the source code.
The "Redistribution" paragraph is the most interesting for you.
To sum up this paragraph in easier terms:
1. Add the Apache 2 license in the source code & somewhere viewable by your end users
2. If you modify a source code which was under the Apache 2 license, you still need to include the Apache 2 license at the top of this source code add write that this file has been modified
3. Do not delete the original copyright notices
4. If a NOTICE file was included in the original code, redistribute it in the way you prefer (for example a popup in the third parties notice menu)
PS: Adding the Apache 2 license text seems stupid for an app that you will never open source. However, it is still useful if you sell your source code to another company. They will be able to check where some original open source software comes from.