It's just a slangy way of emphasizing something. You might say "He got his just reward and then some" when someone really got what was coming to him. Imagine whatever is happening having some more added to it.
They mean that the expression to the left of the sign is greater than or less than (as appropriate) the expression to the right of the sign.
The expression, "is at most" is stating that there is no higher terms than what is being stated.
It means that the number or expression on the left of the symbol is less that the value or expression to the right, or that they are equal.
A mathematical expression is a collection of numbers and variables along with mathematical operators - other than equalities or inequalities.
You might mean an inequality (with 'less than', 'greater than', etc). You might mean an expression.
6x-5
More boats than brains
2c + 3
n-3
expression that is 3 more than a.
I think it mean is 12-4
It is n/2 + 5