In Libre Office what does it means “IF(A24<“B”,$B$13,H24+K24)” .
What does the A24<“B” means and what it returns also what is $B$13 used for?
A24<"B" is a comparison expression, it returns TRUE if the value of the cell content in A24 is smaller than the letter “B”. The entire expression IF(A24<"B",$B$13,H24+K24) evaluates A24<"B" and if TRUE then IF executes the then branch which here it simply returns the value of cell $B$13 (an absolute reference to cell B13). If the comparison result is FALSE then IF executes the else branch and calculates H24+K24 and returns that result.
Simply: if cell A24 contains single character B (or C, D, E,…), then show value of B13 cell, otherwise (if there is “A” or less or empty) sum the values in column H and K (on the same row). AFAIK it works with the first character of the string (A24 in your case), others are ignored.
Dollar sign means fixed position, so you may copy (fill down usually) the formula to another row ($13) or column ($B). The reference will always be the same - B13.
Thats good to know about the fixed reference, I have been struggling with that, So any of the references can be fixed, can I mix them so that some are fixed and others dynamic?
Yes, you can mix them as you like. We don’t speak of “fixed” or “dynamic” though, it is absolute or relative. The difference is when you copy&paste formula cells, absolute references stay the same while relative references are then relative to the position where you paste the formula cell to. See Addresses and References, Absolute and Relative - LibreOffice Help
if cell A24 contains single character B (or C, D, E,…), then show value of B13 cell
That’s not quite correct, the comparison is <"B" meaning smaller than, so if A24 contained B the expression would not show value of B13. Also, it is not limited to single characters, “AB” is still smaller than “B”, as is any number (all numbers are smaller than any string).