Hi Natasha,
The steady state approximation assumes that for an enzymatic process:
E + S --> ES --> E + P
The dES/dt = 0, or the [ES] will remain constant.
This constant concentration of ES is required to derive the Michaelis Menten equation, and therefore reliable Michaelis Menten data.
Please let me know if this is unclear or if I have not addressed your question.
Katt