In binary notation, what is the meaning of the digits after the radix point “.”?

Simple place value. In base 10, you have these places:

… 10³ 10² 10¹ 10⁰ . 10^-1 10^-2 10^-3 …

… thousands, hundreds, tens, ones . tenths, hundredths, thousandths …

Similarly, in binary (base 2) you have:

… 2³ 2² 2¹ 2⁰ . 2^-1 2^-2 2^-3 …

… eights, fours, twos, ones . halves, quarters, eighths …

So the second place after the . in binary is units of 2^-2, well known to you as units of 1/4 (or alternately, 0.25).