A multivibrator that has no fixed, stable state but switches continuously between two states is called an astable multivibrator. Switched is achienved by charging and discharging a capacitor repeatedly. This changes inputs to a NAND gate from high to low and vice versa. The circuit is show below.
If the input to A is high then the output is low, the input to B is low and the output from B is high. The capacit in the circuit therefore becomes charged, increasing the input to B. When this potential is high enough, the output from B switched to low. The input to A then goes low and the output from A goes high. The capacitor then discharges through R until the input to B becomes low enough and the output from B goes high. The process repeats itself with a frequency which depends on the values of the resistance R and capacitance.