A more thorough explanation:

Lords Marchers were lords who lived on the borders of Scotland and Wales. They operated under their own private laws with the permission of the English sovereigns. They were practically independent potentates of a kind very unusual in England.

The laws were eventually abolished by the statute 27 Hen. 8, ch. 26.

From this two consequences flowed. In the first place, there grew up in their jurisdictions a mixture of Welsh custom and English law known as the custom of the Marches. In the second place, although they held of the king, their allegiance sat so lightly upon them that it was necessary to declare in 1354 that ‘all the Lords of the Marches of Wales shall be perpetually attending and annexed to the crown of England, and not to the principality of Wales, in whose hands so ever the same principality be.’

For example, the Lords Marchers were able to operate under their own laws and customs, which were a mixture of Welsh and English law. They were also able to hold their allegiance lightly, which meant they were not always loyal to the English sovereigns.