x,xxx etc. - 'x's are used mainly in the appraisals section, to denote rough estimates of how much a domain is worth. Valuing a domain name is a vague process and prices differ wildly according to the domain, the economy, the end users' pockets, and other factors, so exact valuations cannot be given. So domainers use approximations that may include numbers or x's.
The x's are simply substitutes for tens, hundreds, thousands, etc. For example, we may say a domain is worth $300 - 600... or we may put it another way and say it is worth mid-xxx.
We may say it is worth $50 - 100, or mid to high xx. We might say it is worth $8, or high x.
Here is a general outlay:
x = 0 to 9, since it is a single digit.
Low x = 0 - 3; mid x = 4 - 6; high x = 7 - 9.
xx = 10 - 99, since it covers all double-digit numbers.
Low xx = 10 - 30; mid xx = 40 - 60; high xx = 70 - 99.
xxx = 100 - 999, since it covers all three-digit numbers.
Low xxx = 100 - 300, mid xxx = 400 - 600; high xxx = 700 - 999.
This breakdown continues through x,xxx and xx,xxx and xxx,xxx and x,xxx,xxx etc, referring to thousands, tens of thousands, hundreds of thousands, millions.
All these are vague approximate ranges.