At the start of a syllable block, it acts as a silent place holder (consonant) as it can't start with a vowel (for example 은, 이, 왜) therefore it's silent. However if it's at the end (양, 방) it acts as an "ng" sound... hope it helped!

It's a consonant placeholder, because each syllabe needs to have at least a consonant and a vowel each, when you write them down, but since some only contain a vowel you write down ㅇ where the consonant should be. For example 아 or 우 (if you don't know already, the positions are different depending on the syllabe sontaining a vertical or an horizontal vowel). I hope it helped.