The dinosaurs were a group of land animals that lived during the Mesozoic era for approximately 165 million years, and went extinct (along with many other animals and plants) at the end of this era, in a sudden event known as the Cretaceous-Tertiary Extinction. For most of the time that dinosaurs were around they were the dominant land animals, and they included the ancestors of birds (in modern systems of biological classification, known as "cladistics", birds are technically a subgroup within dinosaurs).
We know a lot about dinosaurs because of the fossils that they left. The best-known fossils are of course the bones, teeth and horns (and sometimes soft body parts such as skin impressions) of the dinosaurs, which are of course displayed in many different museums around the world. Sometimes dinosaurs also left fossils, known as trace fossils (technically "ichnofossils"), which show their activities during life - these include fossilized dung (known as "coprolites"), nests, burrows, toothmarks, and footprints.
Fossilized dinosaur footprints have been found in many locations around the world. Typically they are formed when a dinosaur walked across wet sand (for example), and this was buried by mud or silt. Later these materials turned to rock, and millions of years later we found these impressions of dinosaur's feet embedded within rocks.
Scientists can tell a lot from dinosaur footprints. Apart from the size and shape of the shape of dinosaur's feet that is! They may be able to get an idea of the particular dinosaur's weight, and if there are a series of footprints, how fast it was moving, whether it walked on two or four legs, whether it was travelling in a group, whether the group included both juveniles and adults, and much else besides.
