Basically, the launcher, which you download from the site, can download the mods automatically. This means that when Tekkit updates, your Tekkit will update itself as soon as you start it up. If you haven't changed anything, your Tekkit should be on the Reccomended build of Tekkit. However, there is another version of Tekkit which isn't Reccomended yet. You can switch to this build in the options, look for the Dev build. Don't worry, it's extremely stable for a Development build, so you shouldn't run into any problems.