The becquerel or Bq is the SI unit for radioactive decay, measuring the activity of a substance, and is defined as one nucleus decaying per second (units of s-1). The becquerel is named after French physicist Henri Becquerel, who is known for his work with light, and shared the Nobel prize in 1903 with Pierre and Marie Curie for the discovery of natural radiation.
The ability to measure the activity of a substance is practical and has many applications. For example, measuring the activity of a substance can allow you to determine the half-life of a sample. One useful application involving half-life is radioactive dating, which is used to determine the age of rocks and other materials, which is how the age of the Earth is known.
The becquerel is a very small unit of activity in general applications, so for practical reasons it is commonly prefixed as kilobecquerels (103 Bq), megabecquerels (106 Bq), gigabecquerels (109 Bq) or even terabecquerels (1012 Bq).