An APK is an Android application installation file with an .apk extension, comparable to EXE in Windows and DMG in Mac. It is actually a variant of the Java JAR format, basically a compressed ZIP file which can be inspected with archive file software. It contains all the necessary data for correctly installing and running an app on an Android device.
The letters APK are an acronym of Android Application Package. It is a compressed file format and inside it stores all the components needed for the correct installation and running of an app on a device, from codes to certificates, although in some cases it might be necessary to download new data once the app is installed. In short, it is used to install and distribute Android applications.
These executable files can be installed directly using Google Play, in which case the user is not even aware of them, or from many other sources, such as this very web page. In these cases, the APK files are stored like any other file in your download folder or anywhere else that you place them, from where you can share them in several ways. To open them, all you have to do is click on them, the Android operating system has an app installer capable of working with these files. To install apps that are not from the official store, a settings change first needs to be made on any Android device.
Pick up your device and go to operating system Settings. Go to Security.

Find the option Allow installation of apps from unknown sources or something similar and turn it on.

It is also recommended that you turn off the Google Play Protect function in the Google app store. This function is built into the operating system and constantly analyzes device content in search of malware and it could block installation of apps from external sources, even if they are safe.
APK size can vary from just a few megabytes up to gigabytes, depending on the type of app. As far as security is concerned, Google Play APKs go through a Google filter, although it is not always 100% reliable. The APKs that you can download from Malavida have been tested and analyzed using dozens of different anti-virus software programs to ensure that they do not contain viruses or malware.