Sn2 reaction

 In this reaction 

Nucleophile substitute halogen.... 

Lets see the mechanism

Firstly

This is a one step reaction there is no intermediate like carbonation or Carboanion

Basically in simple language

 -A nucleophile comes

And attack from Backside And the leaving group leaves.. 

Backside attack is happening because if attack will happen from front side there is steric hindrance or electronic repulsion between electron clouds💨 of  leaving groups and nucleophile

After this a invasion of configuration happens

See 👀 the image

As this reaction happens in one step 

We need good leaving group 

And strong nucleophile for this reaction to be happen

Let's see animation of that 

Thnks to university of Surrey this is only for information