With MIT you can create your own QR code and email to someone and they can take their phone and scan it and your app will be downloaded and installed, after they click that they accept programs from an unknown source (not from the store).
Here is what Wikipedia says about QR code: QR code (abbreviated from Quick Response Code) is the trademark for a type of matrix barcode (or two-dimensional barcode) first designed for the automotive industry in Japan. A barcode is a machine-readable optical label that contains information about the item to which it is attached. A QR code uses four standardized encoding modes (numeric, alphanumeric, byte/binary, and kanji) to efficiently store data; extensions may also be used.
Wikipedia goes on to say: A QR code consists of black squares arranged in a square grid on a white background, which can be read by an imaging device such as a camera, and processed using Reed–Solomon error correction until the image can be appropriately interpreted.
On your Android phone you can download a QR code reader program (there are several of them to choose from) and use the camera lenze to read QR codes.
Create Your Own!
Try scanning this barcode above. It takes you to the homepage of this site! I created this easily by going to goqr.me The one below is just some text. At this website you can write up to 300 characters maximum when creating a text QR code.