Can we send multiple signals in same core at the same time in the fiber optics? If yes, how?
This is somewhat oversimplified, but it gets the basics right.

Think of a beam of white light shining into a prism. The output will be a rainbow, which means that the different colors exit the prism at different angles. 

A prism will work just fine "in reverse". That is, if you take a series of lasers of different colors, place them where their colors appear in the spectrum from the prism and point each one back into the prism, they will combine to form a multi-colored beam. Send this beam into another prism, and it will split the beam back into its' separate colors. And that's the basic idea.

The details are different, the biggest change being that nobody uses prisms, but dichroic filters instead.