Atoms have a dense positive nucleus sitting in the exact center. Electrons rapidly orbit this central nucleus in several distinct layered shells. The nucleus constantly pulls inward on all these negatively charged electrons.
The negative electrons also powerfully repel each other at the same time. Electrons in the inner shells sit very close to the dense nucleus. These inner electrons form a repelling wall around the atomic center.
Electrons in the outer shells sit much farther away from the central nucleus. The inner electrons constantly push outward against the distant outer electrons. This outward push cancels out some of the strong pull from the nucleus.
We say the inner electrons shield the outer ones from the full positive charge. The outer electrons only feel a smaller fraction of the total nuclear pull. We formally call this reduced inward pull the effective nuclear charge.
This shielding idea explains many important patterns found on the periodic table. As you move right across a row, atoms actually get physically smaller. The actual nuclear charge increases as protons are added to the nucleus.
The inner shielding layer stays exactly the same across that entire row. The stronger net pull shrinks the whole atom into a tighter shape. A common student misconception is that outer electrons perfectly shield each other.
In reality, electrons floating in the exact same shell barely shield each other. This shielding concept also heavily applies in magnetic testing like NMR spectroscopy. In this advanced test, thick electron clouds physically shield chemical nuclei from strong outside magnetic fields.