There are some walls in my Math formula. |a|

What does it mean if a variable in a Mathematics formula is surrounded by two vertical lines?

According to WolframAlpha "|z| is the absolute value of z."

I guess whether I take |z| or |a| doesn't really matter, so I'll continue with |a| as it seems more common.

In laymen terms this the statement from WolframAlpha means we're taking the actual value from a while disregarding the algebraic sign (+ or -).

So if a = -2.5 then |a| represents the value 2.5

If a = 3.14 then |a| equals the value 3.14

You can see the two | | surrounding the variable name as some kind of programming function that returns the distance from 0 to a. In-fact it exists in languages like C# as the {Math.Abs(value)} function. But even if this function wouldn't exist in your programming language of choice you could implement it as sqrt(a^2) (sqrt is the square-root). Making a squared gets rid of the algebraic sign (22 and -2-2 are both 4). And the square root reverses the effect of the quadration without gaining back the algebraic sign.

In mathematical terms |a| you may come across this notation described ast the absolute value or modulus of the real number a.

In case you're looking for a more exhaustive definition, I can recommend this 3 part video by Eddie Woo.