What is Tenant in Office 365?

I'm new to Office 365 and SharePoint, while I'm trying to create a SharePoint site using CSOM in .NET I've used Tenant like (var tenant = new Tenant(clientContext);) can someone explain what "Tenant" exactly is and what is the use of it in here. When I searched for it I learnt something like Tenant ID which Unique for each Company, but Tenant ID and Tenant in CSOM is different right? and also What is ClientContext? in my code I have used both ClientContext and Tenant to create a SharePoint Site.

using System;
using System.Security;
using Microsoft.Online.SharePoint.TenantAdministration;
using Microsoft.SharePoint.Client;
namespace CreateSiteCollections
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Program Started!");
            //Opens the Admin URL
            using(ClientContext tenantContext=new ClientContext("https://developer19-admin.sharepoint.com/"))
            {
                //Authenticating with Tenant Admin
                SecureString passWord = new SecureString();
                foreach (char c in "passCode1".ToCharArray())
                    passWord.AppendChar(c);
                tenantContext.Credentials = new SharePointOnlineCredentials("kailash@developer19.onmicrosoft.com", passWord);

                var tenant = new Tenant(tenantContext);

                //Properties
                var siteCreationProperties = new SiteCreationProperties();

                //New-Site URL
                siteCreationProperties.Url = "https://developer19.sharepoint.com/sites/codesite";

                //Titie of the Root Site
                siteCreationProperties.Title = "Coded Site";

                //Login Name
                siteCreationProperties.Owner = "kailash@developer19.onmicrosoft.com";

                //Template Copied from Team Site
                siteCreationProperties.Template = "STS#0";

                //Storage Limit in MB
                siteCreationProperties.StorageMaximumLevel = 100;

                //UserCode resourse Points Allowed
                siteCreationProperties.UserCodeMaximumLevel = 50;

                //Creates Site Collection
                SpoOperation spo = tenant.CreateSite(siteCreationProperties);

                tenantContext.Load(tenant);

                //IsComplete to check if provisioning is Completed
                tenantContext.Load(spo, i => i.IsComplete);

                tenantContext.ExecuteQuery();

                while(!spo.IsComplete)
                    {
                    //Waits 30 Sec and tries again
                    System.Threading.Thread.Sleep(30000);
                    spo.RefreshLoad();
                    tenantContext.ExecuteQuery();
                }
                Console.WriteLine("SiteCollection Created.");
            }
        }
    }
}

A Tenant is the Organisation or the Company. It's the "instance of Office 365" that is unique to your user base.

While it's not necessarily a single domain name, because a Tenant could have multiple domain names, that's one way to think of it.

> A tenancy in Office 365 refers to the full Office 365 suite attached
> to a domain. When Office 365 is set up, it creates a tenancy to store
> all the data for Office 365 including things like SharePoint, OneDrive
> and Yammer. This allows all of your organisations data to sit in the
> same environment and be moved around within the tenant with ease.