When you download an app, you'll likely have to authorise the app to access personal information such as your contacts, location or your documents. The apps may need access to this information to help their systems run properly. If you want your weather app to forecast the weather in your location, for example, it makes sense that it requests access to your location. However, the app may want access to your information so that it can sell it to third-party companies. This could lead to a number of spam calls or potential cyber-attacks.
You need to be aware of what information you allow your apps to collect to protect your online privacy and security. Try to read the Terms and Conditions before you download an app. This will help make sure you are aware and comfortable with the level of personal information that you are sharing.