Does the graph mean that if pwm signal below 1500 us provided the thrusters will move in one direction and if more than 1500 us provided then it would rotate in opposite direction?
Yes, that’s correct, at least with the Basic ESC and most bidirectional ESCs.
@adam Then how will the thrusters react with single direction esc? Will it consider 1000 us as the low and 2000us as the highest signal like done in quadcopters or anything different?
That depends on the ESC, but yes, that would be true for most single direction ESCs. The minimum and maximum throttle, neutral, and forward reverse directions are arbitrary, and can be set to whatever PWM input values the programmer wishes to set in the firmware. The standard for most unidirectional ESCs is as you say.