An r-selected species is one that reproduces quickly.
The lowercase r is used to represent growth rate in ecological equations, and so is also used to represent species with a high reproductive rate. These species try to fill all available niches in an environment. Examples of r-selected species are many weeds such a dandelions, insects such as house flies and cockroaches, mice, passenger pigeons, and most bacteria.
r-selected species tend to be small in size, have large broods with high fatalities, and live in unpredictable environments where the probability of long term survival is minimal. They may not live longer than is required to mate and reproduce.
r selected species are also known as r strategists and opportunistic species. They practice big-bang reproduction, AKA semelparity.
r-selected reproduction is the opposite of K-selected reproduction, in which a few young are cared for over a long period of time -- quality over quantity.
Both r-selection and K-selection are comparative terms. Most animals are somewhere along the endpoints of this continuum. Cats are r-selected compared to humans, and K-selected compared to cockroaches.