Question

what is mantisa?

Answer 1

it's the fractional part of a decimal number.

.123 is the mantissa of 61.123

It is often used when describing logarithms base 10.

If the mantissa is .3010

log 2 = 0.3010
log 20 = 1.3010
log 200 = 2.3010

the mantissa provides the basic number, from 1 to 10.
The characteristic is the power of ten to multiply by.