The concept you're referring to is known as the Heisenberg uncertainty principle, a fundamental principle in quantum mechanics. According to this principle, there is an inherent limit to the precision with which certain pairs of physical properties, such as position and momentum (or velocity), can be simultaneously known.
The uncertainty principle states that the more precisely you try to measure the position of a particle, the less precisely you can know its momentum, and vice versa. This means that it is impossible to simultaneously measure both the position and velocity of a particle with arbitrary precision.
The reason behind this uncertainty is that at the quantum level, particles do not possess definite positions or velocities in the same way as macroscopic objects in classical physics. Instead, particles are described by wavefunctions that represent a range of possible positions and momenta. When a measurement is made, the wavefunction "collapses" into a specific state, but the uncertainty principle limits how precisely we can know the values of certain pairs of properties.
This does not mean that an object is physically in two places at once, but rather that its properties exist in a superposition of possibilities until a measurement is made. The act of measurement causes the system to "choose" one of the possible states, and the other possibilities become inaccessible.
It's worth noting that the uncertainty principle applies to certain pairs of complementary properties, but there are other properties, such as energy and time, for which the uncertainty principle takes a different form. In any case, the uncertainty principle is a fundamental aspect of quantum mechanics and sets a limit to the simultaneous knowledge of certain properties of a particle.