What's a quasi-moon? Essentially, it's an asteroid that's entered Earth's orbit and is subsequently affected by the planet's gravitational pull. It starts to look like it's orbiting the Earth as it is close to the planet, much like the Moon.
In reality, its orbit is geared towards the Sun. At the moment, there are four quasi-moons associated with the Earth, the most recent of which was just discovered in July.
It's called 2014 OL339 and it joins 2004 GU9, 2006 FV35 and 2013 LX28 to complete the list of quasi-moons "orbiting" the Earth. Quasi-moons show orbital resonance with the planet, closely matching the path the Earth is taking as it orbits the Sun. It will also take the 2014 OL339 about a year to orbit the sun, much like the Earth.
On its own, 2014 OL339 has an unstable orbit. When it got close to the Earth, the asteroid's path stabilized, characterized by a "wobble" as it circled the planet.
2014 OL339 was discovered on July 29 by Fardi Char from the Chilean University of Antofagasta, appearing to have a size of between 90 and 200 meters. Since September, 2014 OL339 has been observed for 27 times, with observation arcs of 36 days. It entered orbital resonance with the Earth about 775 years ago and will continue for the next 165 years, approximating the duration of its co-orbital episode to be 1,000 years.
In a study, C. de la Fuente Marcos and R. de la Fuente Marcos explore the "dynamical evolution of the recently discovered Aten asteroid 2014 OL339 to show that it is currently following a quasi-satellite orbit with respect to the Earth."
The study further notes that 2014 OL339 is "quite chaotic and together with [2004 GU9] are the most unstable of the known Earth quasi-satellites." And while the quasi-moons 2004 GU9, 2006 FV35 and 2013 LX28 demonstrate Kozai-like dynamics, 2014 OL339 so far does not.
Having four detected so far puts the Earth in at second on the list of planets with the most number of quasi-moons. Jupiter tops that list with six.
Astronomers know that there are far more circling the planet and the rest of the galaxy but quasi-moons are very hard to spot given their size. In fact, it wasn't until 2004 that the first for the Earth was detected.