Parton distribution functions (PDFs) are fundamental quantities in particle physics that describe the internal structure of protons, neutrons, and other hadrons. They represent the probability of finding a particular parton (quark or gluon) with a given momentum fraction inside a hadron. PDFs are essential for calculations in high-energy particle collisions, such as those performed at particle accelerators like the Large Hadron Collider (LHC). They are used in theoretical models to predict the outcomes of scattering experiments and to analyze data collected from these experiments. PDFs are determined experimentally through a combination of data from particle collisions and theoretical calculations based on the principles of quantum chromodynamics (QCD), which describes the interactions between quarks and gluons. Overall, PDFs play a critical role in our understanding of the structure of hadrons and in testing the predictions of the Standard Model of particle physics.