Spacetime. We are used to thinking of our world as having 3 space dimensions and 1 time dimension. From the theory of special relativity we know that space and time are intimately linked. Therefore we combine them into a 4-dimensional spacetime (or 3+1 dimensional spacetime to be precise). Then the trajectory of a moving particle in this spacetime is an invariant object called a 'worldline'. How we measure the space and time dimensions amounts to a preferred set of coordinate axes which depends on our frame of reference. We generalize to higher dimensional spacetimes simply by adding more space dimensions (e.g. 10-dimensional spacetime has 9 space dimensions and 1 time dimension).

We are used to thinking of fundamental particles (like electrons) as point-like 0-dimensional objects. A generalization of this is fundamental strings which are 1-dimensional objects. They have no thickness but do have a length. Strings can be open or closed. As they move through spacetime they sweep out an imaginary surface called a worldsheet. These strings have certain vibrational modes which can be characterized by various quantum numbers such as mass, spin, etc. The basic idea is that each mode carries a set of quantum numbers that correspond to a distinct type of fundamental particle. This is the ultimate unification: all the fundamental particles we know can be described by one object, a string! [A very loose analogy can be made with say, a violin string. The vibrational modes are like the harmonics or notes of the violin string, and each type of particle corresponds to one of these notes.]