expression

The period in the expression you posted is simply an abbreviation for 0.5

So in your wiggle example, where the first parameter (.5) represents how many times the wiggle value should change per second (frequency) it's asking for a new wiggle value every two seconds (0.5).  If you set the value to 1, it would return a new value every second.  If you set it to 2, it would return two new values per second.  If you set it to .25 or 0.25, which is one quarter of one, it would set a new value once every 4 seconds.

That might seem a little confusing because of the maths, but the main answer to your question is simply that decimals don't need leading zeroes in an expression, so .5 is just short for 0.5.

The variables in Wiggle work like this:

wiggle(freq, amp, octaves = 1, amp_mult = .5, t = time)

Usually, all you use is the frequency and amplitude. Your example wiggles the property value between 0 and 100 every half second. Did you follow that?

Here's a good explanation of the wiggle operator.

https://www.schoolofmotion.com/blog/wiggle-expression