Under GST law, generally supplier (seller of goods or services or both) collects tax from recipient (buyer/ customer of goods or service or both) and pay to the Government (net of ITC) (but ultimately GST is borne by end customer or consumer that is why it is called indirect tax) this mechanism is known as forward charge.
But in few cases due to administrative difficulties (e.g. supplier is located outside India, large number of small suppliers etc.), GST law has imposed responsibility on recipient of goods or services or both to pay GST by himself directly to the Government not by supplier so this mechanism is known as reverse charge or also referred as reverse charge mechanism (RCM).
Source Taxguru
