​ is the HTML entity for a unicode character called the zero-width space (ZWSP).
“In HTML pages, this space can be used as a potential line-break in long words as an alternative to the
<wbr>tag.”- Zero-width space – Wikipedia
The <wbr> tag also works, as mentioned by Aaron’s answer. I think I prefer the HTML entity over the tag because the entity seems simpler: unicode handles it, not the web browser.