tab stop in a table cell

Vicki: what is: As ␉ is used for Table navigation, to insert a ␉ character

␉ is Unicode U+2409 SYMBOL FOR HORIZONTAL TABULATION
used to denote the [Tab⭾] key on the keyboard.
If it's not visible in your view of the forum, it is usually rendered as HT or ^H_T.
␛ is Unicode U+241B SYMBOL FOR ESCAPE
used to denote the [Esc] key on the keyboard.
If it's not visible in your view of the forum, it is usually rendered as ESC or ^ES_C

In a Table, if you just hit ␉, the cursor moves to the next cell. If you hit ␛ followed by ␉, you insert a tab in the current cell.

These ␉ & ␛ codepoints are one of the intended use cases for these Unicode characters, visually representing, for discussion and document purposes, the actual characters generated by those keys:
U+0009 HORIZONTAL TABULATION
U+001B ESCAPE
which are not normally displayed, but executed instead.

In some future that may or may not be near you, any single character can be represented, at final document or web page render, as a keycap, via a trailing:
U+20E3 COMBINING ENCLOSING KEYCAP

Browser support for this can't yet be relied on. I've actually never seen it work {…properly; Firefox tries}, but here they are as a test:
Enclosed tab: ␉⃣
Enclosed escape: ␛⃣

The letter strings Tab⭾ and Esc followed by a U+20E3 can't be relied on at all, because 20E3h is only documented to enclose a single character.