hahaha

In an experiment, the rate of an enzyme-catalyzed reaction increased as the substrate concentration increased, but it leveled off after reaching a certain point. Why is there a steep slope in the beginning? why does it level off when more substrate is added? I need help with this question..i have turned my bio book inside out and i still cant find the answer..i feel like it is soo simple and i am over thinking it..any help at all is appreciated