Download means to transfer a file. You are transferring the file to your phone. Install means to set it up and configure it, so that it works properly and can be opened. The reason it is confusing is because on most phones, there is *one* button which does *both* actions - you press that button and your phone will download the app and then install the app. Most people don't realise two steps are happening. The phone simplifies the process to make it easy.

I can help you to learn Hindi.Can you help me to learn English?

you choose one app and you wanna use is on your phone, the fisrt thing you need "download" this app, and the next "install" it, then you can use it. "download" is the app/software you do not have on your device like your phone or computer, but you want to have it, so you have to download it first. The software installation package is what you download , you can not entre it and use this software through this software installation package. “install” is when you have the software installation package on your device, you press this package then you will see the "install" is proceed , when the proceed finish you can final use this software/app. I hope it can helps you.