Yii use and namespace

use

Standard Yii

This is a keyword which is used to make functionality available to the current class.
E.g. If you add line below
use yii\db\ActiveRecord;
You will be able to create a new ActiveRecord such as
$ar = new ActiveRecord();
Or to create a class which is an extension of an ActiveRecord such as
class Customer extends ActiveRecord
{
}

Your Yii custom components

E.g. If you add line below
use app\models\Customer;
You will be able to add functionality which has been created for this application. In this case a model was created called 'Customer'. From here you can such functionality to your current class as
$query = Customer::find();

namespace

The namespace allows you to create a structured naming convention to retrieve the class you are currently working on.
E.g. If you add lines below
namespace app\models;
use yii\db\ActiveRecord;
class Customer extends ActiveRecord
{
}
This will allow you at a later date to 'use' this class thus
use app\models\Customer;