This new word may look familiar if you have encountered the particle 도 (do).  Fortunately they have almost the same meaning. 또 (ddo) is an adverb that means the same as particle 도 that is– again, also or too.  The only difference is that particles need to be attached to a noun or noun expression, so 또 is independent and may exist as a word in a sentence.  See how these two are used in the following sentence:

• 여기에 쓰레기도 있어요 (Yogi.eh ssuregido issoyo) – There are litters here too.
• 저기에 쓰레기는 또 있어요 (Chogi.eh ssureginueun ddo issoyo) – There are some more litters over there.

Since 도 is a particle there is no need to put markers 가/이 or 는/은 when it is attached to a noun that is subject or topic of a sentence.

또 like the word 그리고 (kurigo) may be used at the start of a sentence which means and further(more), and also or simply and.  You may likewise find is both at the start of the sentence 그리고 또 (kurigo ddo) thus it will then mean ‘and moreover’ or ‘and furthermore’.