Skeletal muscle is made of muscle cells/myocytes.
Each muscle fibre contains numerous myofibrils.
The myofibril consists of repeating alternate light & dark bands which are called a sarcomere.
Thus the basic fundamental unit of striated muscle tissue is the sarcomere.
The sarcomere consists of long, fibrous protein filaments that slide over each other during muscle contraction & relaxation.
It is present between 2 Z- lines.
Around the region of the Z -line, the I band is present.
There exist thick and thin filaments that are myosin and actin respectively.
Just next to the I-band, A-band is present that has thick filaments in it.
Within the A-band, an actin deficient zone is present which is called H-zone.
The sliding of the actin filament past the myosin filament leads to muscle contraction.
Final Answer
Thus, sarcomere gives the striated appearance to skeletal and cardiac muscle fibres. It helps in the relaxation and contraction movement of muscles.