As has been discussed in my recent series of essays on quantum mechanics, an entity's location involves a lot of weird ideas. According to the concept of a probability wave, based on the size of a wave at a given location, an object might be here with some probability or there with some probability. It might even be in two places at once.
Another major topic in quantum mechanics related to this theme is the Heisenberg Uncertainty Principle. Brian Greene, in his book The Fabric of the Cosmos, characterizes the principle as follows:
It says, roughly speaking, that the physical features of the microscopic realm (particle positions, velocities, energies, angular momenta, and so on) can be divided into two lists, A and B... the more precise your knowledge of a feature from one list, the less precise your knowledge can possibly be about the corresponding feature from the second list... As an example, the more precisely you know where a particle is, the less precisely you can possibly know its speed (p. 96).
A common explanation of the Uncertaintly Principle, which Greene cites, but acknowledges as "incomplete," is as follows:
When we measure the position of any object, we generally interact with it in some manner... Even light [for seeing an object to locate it], when bouncing off an object, gives it a tiny push. Now, for day-to-day objects such as the book in your hand or a clock on the wall, the wispy little push of bouncing light has no noticeable effect. But when it strikes a tiny particle like an electron it can have a big effect: as the light bounces off the electron, it changes the electron's speed... (pp. 96-97).
Greene later cautions that:
The explanation of uncertainty as arising through the unavoidable disturbance caused by the measurement process has provided physicists with a useful intuitive guide as well as a powerful explanatory framework in certain specific situations. However, it can also be misleading. It may give the impression that uncertainty arises only when we lumbering experimenters meddle with things. This is not true. Uncertainty is built into the wave structure of quantum mechanics and exists whether or not we carry out some clumsy measurement (p. 98).
Although I tend to stay away from mathematical aspects on this website, the Uncertainty Principle is embodied in an equation. As John Gribbin notes in Quantum Physics: A Beginner's Guide to the Subatomic World:
...the uncertainty in position multiplied by the uncertainty in momentum is always greater than Planck's constant. We do not notice Planck's constant in everyday life because it is so small, but it is very important for an electron (pp. 33-34).
In other words, the uncertainty in these physical features can never be reduced to zero and must exceed a certain value (Planck's constant), albeit a small value.
The Uncertainty Principle also played a role in the famous debates between Albert Einstein (critiquing quantum mechanics) and Niels Bohr (defending it), each of whom was joined by other colleagues.
As I read Greene's discussion on pages 99-103, Einstein's position (with colleagues Podolsky and Rosen) was that particles do simultaneously possess definite positions and velocities, but the inability to determine both was a limitation of quantum mechanics. Bohr (joined by Pauli and perhaps others) argued that, whether or not particles really do have simultaneously measurable positions and velocities, all that matters to science is what can be measured.
An essay by Rochelle Forrester probes aspects of the Bohr-Einstein debate in greater detail.