The land referred to as the Indian subcontinent is a tectonic plate that began to separate itself from other surrounding slabs of rock (or plates) millions of years ago.
The movement of that plate changed the landscape, and formed the Himalayas, the world's most elevated mountain range; home to Mount Everest, the world's tallest mountain.
The subcontinent land itself is a peninsula that extends south into the Indian Ocean. Geographers refer to it as a subcontinent because of its size, but in reality, it's not large enough to be considered an individual continent.
It includes all of India, as well as Bangladesh, Bhutan, Nepal, Sri Lanka and (parts of) Pakistan, and there seems to be no definitive agreement on the exact landmass content regarding Pakistan.