Do you know what this regular expression mean?

yes I do know what regular expression mean
regular expression is a rule that describes what kind of text you’re looking for

I mean, this particular one [A-Za-z\s]{2,}.

It does not quite match the description of the title attribute value:
Please enter your full name with at least two letters.

I don't understand the question, and i don't see the challenge you're trying to address here

What names does the pattern [A-Za-z\s]{2,} match?

The value of the title attribute is "Please enter your full name with at least two letters.". Does the "letters" include space characters?

I know what you mean now, I mean in the case there are two ways to fix it, either change the regex to enforce full name as in the description or change the description from full name to just name
I will do the later as full name isn't necessarily required but want to enforce characters to the minimum

To only enforce minimum number of characters in the input, there is a HTML attribute for that purpose.

Regex is more suitable if we need to ensure the input matches certain pattern. You can explore regex later.