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What is a 'layshaft' in a manual trans.?

A previous post suggested I might have a bad layshaft or layshaft bearing. My manual trans.-6 spd Nissan Frontier 30k mi. 2wd makes a whine in 1st thru 3rd gears while cruising 20 to 30mph but no whine in 4th thru 6th. I go to the dealer Friday to check it out. What is a gearbox layshaft?? Thanks.

A transmission has 2 main shafts inside of it; a mainshaft and a countershaft. The countershaft is sometimes called a layshaft.

Power is fed into the transmission through the mainshaft and fed out of the transmission by the countershaft.

I would think a countershaft problem, if it exists, would be present in all gears; not just the lower ones. A subtle whine in 1st or 2nd gear may also be normal as those 2 gears often have straighter cut gear teeth. Third gear and higher have gear teeth with more of a helical cut to them which allows them to run quieter.