If we say

If  it rains then I use an umbrella

then it rains implies I use an umbrella

it rains  →  I use an umbrella

(I may use an umbrella even if it does not rain, for example to protect myself from the sun or something.)

But if we say

Only if  it rains then I use an umbrella

then the implication is actually going in the other direction,
namely I use an umbrella implies it rains

I use an umbrella  →  it rains

(It might rain even if I do not use an umbrella, for example if I like to walk in the rain without umbrella or maybe I forgot the umbrella.)

Now, if we combine the two, and say

If and only if  it rains then I use an umbrella

then what we say is that if one, then the other.

it rains  ↔  I use an umbrella

(The situation will either be that both are true or that both are false, there is no other possibility.)