Complex sentence: Firth is the Lowland Scots word often used to denote a large sea bay in Scotland, which may be part of an estuary, or just an inlet, or even a strait (as in the case of the Pentland Firth).
Split into simpler sentences: Firth is the Lowland Scots word often used to denote various coastal waters in Scotland. It is usually a large sea bay, which may be part of an estuary, or just an inlet, or even a strait (as in the case of the Pentland Firth).