The 555 timer is capable of being used in astable and monostable circuits. In an astable circuit, the output voltage alternates between VCC and 0 volts on a continual basis.
By selecting values for R1, R2 and C we can determine the period/frequency and the duty cycle.
The period is the length of time it takes for the on/off cyle to repeat itself, whilst the duty cycle is the percentage of time the output is on. ie. T1/T.
In this type of circuit, the duty cycle can never be 50% or lower.