A unit is a stake of whatever you want it to be. The unit may be worth a fixed dollar amount or a fixed percentage of their bankroll. Some people flatbet and call this a unit while others vary their betsizes and use a scale of units to judge what the bets are worth.
Unit is just a measure used on the forum to explain how much one is risking on a given bet for others to view and compare.
1 unit play being low end rated play, 5 unit play being a higher rated play...some posters will use higher unit values.
Problem is people vary greatly in how they use 'units'. One may use 1 to 5 units while another may bet 0.5 to 2 units. Some may just flatbet 1 unit. Others maybe have an average bet of 10 units.