Speed Control of Brushless DC Motor
Callback function must be data send from server to the ESP.
Payload is the data byte sent from server.
Length is the size of data in bytes.
BTOF is basically byte to float converter which converts all the data byte received into single floating variable and return that floating point value.
ECS.write must be the function that write the value to the servo motor controller H-bridge IC. It belongs to servo library.
Value range that can be mapped into this ECS.write is between 0-180 so map function will use that will convert the floating point data received from btof function.
Basic definition of map() function: 


Map function maps a number from one range to another. That is, a value of fromLow would get mapped to toLow, a value of fromHigh to toHigh, values in-between to values in-between, etc.

Does not constrain values to within the range, because out-of-range values are sometimes intended and useful. The constrain() function may be used either before or after this function, if limits to the ranges are desired.

Note that the "lower bounds" of either range may be larger or smaller than the "upper bounds" so the map() function may be used to reverse a range of numbers.

Syntax:

map(value, fromLow, fromHigh, toLow, toHigh)


Then ECS.write(value) will write that mapped value to servor motor controller H-Bridge.