Complex sentence: Once the day's pivot point has been calculated, traders turn to the calculation of the initial resistance (R1) and support (S1) levels which assumes that trading will continue pretty much in the same range as the previous day.
Split into simpler sentences: Once the day's pivot point has been calculated, traders turn to the calculation of the initial support and resistance levels (R1 and S1). It is assumed that trading will, for the most part, continue in the same range as the previous day.