What does "the world's strongest currency" title mean for investors? Kuwaiti Dinar shows up as the most expensive in the world. Does that mean I better invest in KWD (or the safe USD) instead of my home currency (IDR)?

A little background: I am just an ordinary person without economics / finance background and hope to learn from the experts here. My home currency never fails to show up in the top 5 least valuable currencies in the world. Economic crises that come too often and stay too long make me wonder if I make the right decision to put all of my retirement savings in local equity mutual funds.
The strong currency understanding in your question does not have any interpretation that would be an economic factor that should effect investment, or any other decision. It is merely units.

It answers the question of '1 US dollar buys least of which currency?'. It also answers the question of '1 Colombian peso buys least of which currency?', or any other benchmark currency of your choice.

The Kuwait government could say that they decided to replace KWD by KWD2 and 1 KWD would equal 1000 KWD2. Then everything that has a price of $x$ KWD would now have a price of $1000x$ KWD2. Now, your US dollar would buy a lot of KWD2, but your investment decision would not be effected by this change because there has been only a change in units, and no other intrinsic differences.