Push-ups target the chest, but they're actually not the best exercise for isolating the pecs, as they also require substantial engagement of the core, shoulders, and triceps. The reason they make this list, though, is because they're such a versatile movement. Namely, push-ups require zero equipment, they can be done anywhere, and there are lots of modifications and variations to make them harder or easier depending on your fitness level. For instance, you can make them more difficult by performing them with a suspension trainer or while wearing a weight vest, and you can make them easier by putting your knees on the ground or doing them with your hands on a bench or wall. Plus, they're a great way to get warmed up for more serious pec-focused exercises.
The basic push-up is probably an exercise you're familiar with. Start on your hands and knees, your palms under your shoulders, but slightly wider than shoulder-width apart. Step your feet behind you, balancing on your toes, so your body forms a straight line from heels to head. Tighten your core to keep your hips flat, then bend your elbows, lowering your chest toward the ground, your elbows pointing back and out to create a 45-degree angle with your torso. When your chest is a couple inches from the ground, press through your palms and return to the starting position.