The Heisenberg Uncertainty Principle states that certain pairs of properties, like position and momentum, cannot both be measured with perfect precision at once. The more precisely you pin down where a particle is, the less precisely you can know how fast it is moving, and vice versa.
This is not a limitation of our instruments. It is a fundamental feature of nature. A particle simply does not have both a perfectly defined position and a perfectly defined momentum at the same time, the two are linked by the wave nature of matter itself.
Werner Heisenberg formulated this in 1927, and it reshaped what physicists thought was even possible to know about the world. It explains why electrons do not spiral into the nucleus of an atom, and it sets a hard floor on how precisely any experiment can ever measure certain quantities.