In the astable Multivibrator mode the circuit wil keep re-triggering it self resulting in a pulse train The circuit for the 555 in astable Multivibrator mode looks like this
In this case the capacitor charges and discharges between 1/3 Vcc The 555’s output will the high while charging and low while discharging The capacitor charges and discharge at different rate it has to charge through Ra and Rb but it only discharges through Rb