Answer:
In simple words , monophonic means one note at a time while polyphonic two or more different notes at the same time.
For example:
If one person is singing a tune then this is monophonic .
If two (or more)persons singing the same melody(in Unison) this is still monophonic.
If two (or more)persons singing the same melody but in different intervals (e.g. Octaves) then this is polyphonic(very simple but polyphonic).
Now , if two (or more)persons singing different melodies that are combined together and being perceptible as one total piece of music then you have the full definition of polyphonic music.
(This composing technique was developed most in the Baroque age by composers such as J.S.Bach)
There is also the term "homophonic music"which defines the most common possibility for nowdays that a solo melody("monophonic")is being accompanied by an instrument or an ensemble of instruments(a band).