Q:

Wy we need to put e.target.name in square brackets []?

Why when I use form elements I got to put e.arget.name in brackets  ? 
Here my code : 
onChange (e) {
   this.setState({ *[e.target.name]* : e.target.value});
}

(...) 
  <div>
     <label htmlFor=""> Title : </label>
     <input type="text" name="title"  onChange={this.onChange} value={this.state.title} />
  </div>
</form>

A:

This is to dynamically update object property (when the name of the property is unknown upfront but runtime). This way you could have multiple React inputs having a different name property and using the same onChange handler to update part of the state.
Related to this.
Instead of this:
<input type="text" name="title" onChange={this.onTitleChange} value={this.state.title} />
<input type="text" name="address" onChange={this.onDescriptionChange} value={this.state.address} />
<input type="text" name="description" onChange={this.onAddressChange} value={this.state.description} />

onTitleChange (e) {
   this.setState({ title : e.target.value});
}
onAddressChange (e) {
   this.setState({ address : e.target.value});
}
onDescriptionChange (e) {
   this.setState({ description : e.target.value});
}

We can write just one handler like you presented:
<input type="text" name="title" onChange={this.onChange} value={this.state.title} />
<input type="text" name="address" onChange={this.onChange} value={this.state.address} />
<input type="text" name="description" onChange={this.onChange} value={this.state.description} />

onChange (e) {
   this.setState({ [e.target.name] : e.target.value});
}

