Question

What so you mean by Addition reaction.

Answer 1

Addition Reaction

As the name suggests, It has something to do with addition, right? :)

Addition reaction is the reaction where two molecules combine together to form a larger molecule, provided no other product is used. 

It's like 1+1=2
Two 1 when added gives you 2. 

Therefore, Addition reaction is nothing but the addition of 2 smaller molecules to form 1 larger molecule.

I hope, It's clear to you now? :)

Answer 2

the process by which the unsaturated gain hydrogen's and become saturated hydrocarbons is know as addition reaction.
you can easily memorize it.the word addition states the adding oh hydrogen atoms to unsaturated hydrocarbons which have double or triple bond,gets hydrogen and turns to saturated ones.

use of this process:
vegetable oils which have long unsaturated hydrocarbons are much healthier than the animal fats which are saturated ones. 

in many industries,they use this process known as hydrogenation.this is where the unsaturated carbons are changed to saturated ones with the help of catalysts( it is a substance that wont affect the chemical reaction but make a path for the reaction to take place ) that help the saturated carbons gain hydrogen and transfer it to saturated ones.
the reaction,


R     R                                         H   H
 I      I      nickel catalyst             I    I
C== C    -------------------------> R-C - C-R
 I      I            adds H 2                I    I
R     R                                         H   H

hope it helps!!!!