What does it say in the image?

It says 'minimum'.

Hard to read? Medieval scribes thought so too.

That's why they invented the dot on the i.

This way, you could at least see which strokes represented vowels - and that helped a lot.

For similar reasons, the letter j was invented.

Two ... 2/

2/ ... consecutive dotless ı's looked a lot like the letter u. A Middle Dutch word such as 'dııc' could be mistaken for 'duc'.

That's why the second ı was lengthened to ȷ: 'dıȷc'.

It originally represented the long ee sound as in English 'freeze': [iː].

However, in the 14th century, ... 2/

3/ ... it started to become a diphthong in certain regions, initially close to what you hear in Cockney 'me': [ɪj].

Later, it became similar to English ay in 'may'.

In certain regions in The Netherlands and Flanders, it eventually became the diphthong heard in 'my'.

Nowadays, ij is ... 3/

4/ ... considered a digraph.

Some even insist it's one single letter (and they can be very vocal about it).

At any rate, ij is always capitalised as a whole: 'ijs' becomes 'IJs' if it's the first word of a sentence.

On signs like the one below, both letters are often put in the same box.

@yvanspijk Here's a special ij key on my prewar Dutch typewriter, shared with the equally Dutch florin sign.
@edgeofeurope Wauw, de Nederlandse toets der Nederlandse toetsen!

@yvanspijk ofwel, de Nederlandste toets

@edgeofeurope

@knuppelbeer @yvanspijk @edgeofeurope My typewriter in the early 80s also had this as a letter. / Mijn schrijfmachine in de vroege 80er jaren had het ook als een special toets.

Unicode also still considers it as a single letter for the Dutch locale, and sorts ij and íj' between ï and j: https://unicode.org/cldr/charts/42/summary/nl.html

Locale Data Summary for Dutch [nl]

@yvanspijk @edgeofeurope

The only thing that could make it more Dutch would be the addition of the ‘krul’. (But I suppose no typewriters had that)

https://en.wikipedia.org/wiki/Flourish_of_approval?wprov=sfla1

Flourish of approval - Wikipedia

@edgeofeurope @yvanspijk On keyboards for Monotype type-setting machines, there were keys for every ligature, including ff, fl, fi, ffl, ffi etcetera.
Some fonts have these ligatures automatically, but when they were typeset with lead, they had to be specially selected.
@edgeofeurope @yvanspijk My Rheinmetall (early 60's) also has one.