The Android SDK (software development kit) is a set of development tools used to develop applications for Android platform.
This is a huge file! This installer alone is 1.4gb in size; extracted content after installation is 4.57gb! This being said, please allow some time for it to be extracted. Be aware that this should be installed on a drive with at least 8gb as you're probably using this with Android Studio Portable. However, I would recommend 16gb and up so there will be room for more content as you work.