{{Hsp}} produces the "hair space" character (Unicode hexadecimal alias   / U+200A), a space character that is slightly thinner than the "thin space" character (see below and Whitespace character).

Comparisons

rætta
Name / size →Instance← Example code and ... output
none →← Some text<ref>...</ref> Some text[1]
Hair Some text{{hsp}}<ref>...</ref> Some text[1]
Thin Some text{{tsp}}<ref>...</ref> Some text[1]
Standarda → ← Some text&nbsp;<ref>...</ref> Some text [1]

 a Provided here by a non-breaking space (&nbsp;).

See also

rætta