Enzymes are biological catalysts that either enhance or delay particular chemical mechanisms in a biological system. Enzymes are complex molecules (proteins) that are present in the body in certain amounts, which when triggered, performs a function to helps the body as a whole, without itself being converted, altered or used up.
This can be a laboratory method in determining the activity of enzymes in a particular reaction. As the enzyme is placed in a container with compounds A and C, chemical reaction could happen. The equations can be represented as:
A + C ---enzyme---> product/s (A)
A ---enzyme---> product/s (B)
C ---enzyme---> product/s (C)
A and C are called substrates and the resulting compound (or compounds) is the product. Based on your query, we are not sure if A and C are the substrate of the enzyme, it can be the A alone or B alone (equations B and C). Also, there is a possibility that those products to be formed are more than one compound. The time is being recorded in order to determine the rate of reaction. Any changes in the reaction vessel are carefully recorded with time.