The reaction that takes place in the stroma is collectively known as the Calvin Cycle. The stroma is responsible for the 2nd stage of photosynthesis where ATP and NADPH are used to capture and reduce carbon dioxide in the plant cell.
It is a fluid-like organelle that surrounds the grana within the chloroplast in plant cells. The stroma is also responsible for storing chloroplast DNA and chloroplast ribosomes where molecular processes like replication and transcription/translation of chloroplast proteins occur.