I'm using Trailhead to learn something about Lightning, since we are refactoring form Classic.
The tutorial had me created a new App using the Setup > Home > App Manager > New Lightning App path.
I created my app, but when I attempt to access it, I cannot see it in the App Launcher view. 
I can see it in the App Manager, so I know it's there. 
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The directions are:
From the Home tab in Setup, enter App in the Quick Find box, then select App Manager.
Click New Lightning App.
In the Lightning App Wizard, create an app with these parameters.
App nameDelivery Tracker
DescriptionTrack warehouse deliveries.
ImageYour choice!
Use a JPG, PNG, BMP, or GIF image that’s smaller than 5 MB. For best results, upload an image that’s 128 by 128 pixels. Images larger than the maximum display of 128 by 128 pixels are automatically resized.
Primary hex color value#09D4EA
App navigationStandard
Navigation bar items (in this order)Warehouses, Deliveries, Merchandise, Accounts, Contacts, Cases, Reports
Assigned to user profileSystem Administrator
Click Save and Finish to exit the wizard.
Click  to open the App Launcher, and select the Delivery Tracker app.
Check out the new app!
It’s got all the custom branding we gave it: a custom icon in the upper left and the custom color we assigned to it. Because Warehouses is first in the navigation bar, it becomes the landing page for the app. And even though App Launcher was an item available for the navigation bar, we didn’t have to add it. It’s there, accessible by clicking .

Controller
public class lonchangeaccount{
        public id aid{get;set;}
        public boolean render{get;set;}
 
        List<selectoption> options=new List<selectoption>();
        public  List<Contact> lstcont{get;set;}
        public lonchangeaccount(){
                    lstcont=new List<contact>();
           
            }
      
        Map<id,List<contact>> maplstcont=new Map<id,List<contact>>();
        public List<SelectOption>  getValues(){
                options.add(new selectOption('--None--','--None--'));
                for(Account acc:[select id,name,(select id,name,email from contacts)from account]){
                    options.add(new selectOption(acc.id,acc.name));
                    maplstcont.put(acc.id,acc.contacts);
               
                }
                return options;
        }
        public void  Contactlst(){
                        lstcont.clear();
                       if(aid!=null){
                            render=true;
                            lstcont.addAll(maplstcont.get(aid));
                        }
         }
}

VF Page
<apex:page controller="lonchangeaccount">
<apex:form >
         <apex:pageBlock title="Select Account">
         <apex:actionFunction action="{!Contactlst}" name="change"/>
         <apex:selectList value="{!aid}" multiselect="false" size="1" onchange="change();">
                <apex:selectOptions value="{!values}"/>
            </apex:selectList>
            </apex:pageBlock>
            <apex:pageBlock title="Related Contacts" >
          <apex:outputPanel rendered="{!render}" id="balu">
           
              <apex:pageBlockTable value="{!lstcont}" var="c">
             
                      <apex:column value="{!c.name}"/>
                      <apex:column value="{!c.email}"/>
              </apex:pageBlockTable>
             <apex:commandButton value="Next"/>
         </apex:outputPanel>
   
    </apex:pageBlock>
    </apex:form>

</apex:page>