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Straps are usually made of nylon or another fairly strong flexible material (see picture). They are put around the wrists and then wrapped around a barbell, dumbbell, or other bar to aid in gripping on heavy lifts. Essentially they make up for a lack of grip strength, therefore I don't recommend them unless you are doing extremely heavy Olympic style lifts or are very advanced and your grip strength is limiting you on an exercise. Doing exercises without straps will build your grip strength, therefore do them without unless absolutely necessary.