What is meant by the delta equivalent sign?

It is a definition. Sometimes it is used with the slightly different meaning of "equal by definition", to underline the difference w.r.t. "$:=$ " which is the definition itself.

i.e.

$$ a:=3;\\ 5+a \triangleq 5 + 3 = 8 $$

It is also used in physics, to indicate that forces are drawed on some scale, so for example $$1 N \overset{\Delta}{=} 0,1 \; \mathrm{m}$$

It is clear that a force isn't equal to some length, but they are corresponding with the same length. It is the same principle as Saphrosit says, i.e. they are by definition, and in this specific case, equal to each other.