lightning:accordion component groups related content in a single container. Only one accordion section is expanded at a time. When you select a section, it’s expanded or collapsed. Each section can hold one or more Lightning components.

Here is an example of Lightning Accordion. I’m retrieving a list of contacts from Salesforce and populating into the Lightning Accordion.

Apex Controller:

public class AccordionAuraController {
    @AuraEnabled
    public static List<Contact> getContacts(){
        List<Contact> contactList = new List<Contact>();
        contactList = [SELECT Id, Name, Email, Phone, MailingStreet, MailingCity, MailingState, MailingPostalCode, MailingCountry From Contact LIMIT 10];
        return contactList;
    }
}

Lightning Component:

<!--Accordion.cmp-->
<aura:component controller="AccordionAuraController">
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    <aura:attribute name="conList" type="List"/>
    <div class="slds-m-around_xx-large">
        <lightning:accordion>
            <aura:iteration items="{!v.conList}" var="con">
                <lightning:accordionSection name="{!con.name}" label="{!con.Name}">
                    <aura:set attribute="body">
                        <p>Street : {!con.MailingStreet}</p>
                        <p>City : {!con.MailingCity}</p>
                        <p>State : {!con.MailingState}</p>
                        <p>Postcode : {!con.MailingPostalcode}</p>
                        <p>Country : {!con.MailingCountry}</p>
                        <p>Email : {!con.Email}</p>
                        <p>Phone : {!con.Phone}</p>
                    </aura:set>
                </lightning:accordionSection>
            </aura:iteration>
        </lightning:accordion>
    </div>
</aura:component>

Lightning Component JS Controller:

({    
    doInit: function(component){
        var action = component.get('c.getContacts');
        action.setCallback(this, function(response){
            var state = response.getState();
            if(state === 'SUCCESS' && component.isValid()){
                //get contact list
                component.set('v.conList', response.getReturnValue());
            }else{
                alert('ERROR');
            }
        });
        $A.enqueueAction(action);
    }
})

Lightning Test App:

<!--Test.app-->
<aura:application extends="force:slds">
    <c:Accordion />
</aura:application>

Output:

Note: The first section in the lightning:accordion is expanded by default. To change the default, use the activeSectionName attribute in lightning:accordion component. This attribute is case-sensitive. If two or more sections use the same name and that name is also specified as the activeSectionName, the first section is expanded by default.