Supposing that one end of a string is fixed and the force acts at the other end, then the string will be stretched because it will be pulled at both ends. The force that acts at the fixed end, has the same magnitude as the force that acts on the other end, but opposite direction.
The force that acts on the fixed end is called tension and the notation used for tension force is T.
Using the Newton's second law, yields:
`T - m*a = 0 => T = m*a`
`F = m*a` represents the force that acts at one end of the string and `T` represents the tension in string.
Hence, the magnitude of the tension force is the same as the magnitude of the force that acts on the opposite end of the string.