It means the value will only be replaced if it is nil or false. (Correct me if I'm wrong please, I'm also new)

#Case 1:
a = 1
a ||= 10
print a #=> 1

#Case 2:
a = nil
a ||= 10
print a #=> 10

a||= 10 is logically equivalent to: a || a = 10

Let's understand that behaviour. When we put together many or's:
a || b || c || d, it will return true immediatly when the first true value is found.
So in the example above, if 'a' is true, then b, c and d WON'T be evaluated by the or operator.

The same happens here: a || a = 10.

if a is false (or nil) it will evaluate (a = 10) which then changes the value of a.
if a is true (or != nil), then (a = 10) WON'T be evaluated at all.