title: trigger to update a custom field in account when related opportunity is created with same value as in custom field of Opportunity
body: <p>Can someone please elaborate what is mapAccounts.get(opp.AccountId) is getting here <code>MapAccount acc = mapAccounts.get(opp.AccountId);</code></p>
  
  <pre><code>trigger OppTrigger on Opportunity (after insert) {   
      if(Trigger.isInsert &amp;&amp; Trigger.isAfter)
      {
          Set&lt;Id&gt; acctSet = new Set&lt;Id&gt;();
          List&lt;Account&gt; accList = new List&lt;Account&gt;();
          for(Opportunity opp : Trigger.new)
          {
              acctSet.add(opp.AccountId);
          }
  
          Map&lt;Id, Account&gt; mapAccounts = new Map&lt;Id, Account&gt;([SELECT Id, accField__c FROM Account where Id IN :acctSet]);
          for(Opportunity opp : Trigger.new)
          {
              Account acc = mapAccounts.get(opp.AccountId);
              acc.accField__c = opp.oppField__c;
              accList.add(acc);
          }
          Update accList;
      }
  
  }
  </code></pre>
  
  <p>there is no error in this trigger but i need to understand in detail, what <code>mapAccounts.get(opp.AccountId);</code> is getting here</p>