In string theory, the fundamental building blocks of the universe are not point-like particles but tiny, vibrating strings. These strings are considered to be the fundamental entities from which all particles and interactions arise.
According to string theory, these strings are one-dimensional objects, meaning they have length but no width or height. They are incredibly small, on the order of the Planck length (about 10^(-35) meters), which is currently the smallest length scale that can be meaningful in our current understanding of physics.
The strings in string theory are not made up of anything more fundamental; they are the fundamental constituents themselves. They can vibrate in different modes, and the specific patterns of their vibrations give rise to the different particles and their properties. The vibrational modes of the string determine the mass, charge, and other characteristics of the particles that they represent.
It's worth noting that string theory also allows for extended objects called "branes" (short for membranes) of higher dimensions. These branes can have more than one spatial dimension and can play important roles in certain aspects of the theory. However, at the most fundamental level, the primary building blocks in string theory are the strings themselves.