Select some cells and choose Insert > Checkbox. Each cell becomes a box you tick with a click or the Space bar. Behind the box the cell holds a value: TRUE when ticked and FALSE when not.
Because they're values, formulas can use them:
=COUNTIF(C2:C13, TRUE) counts the ticked boxes. TRUE has no quotes: it's a value, not text.=COUNTIF(C2:C13, TRUE)/COUNTA(A2:A13) gives the share done.=SUMIF(C2:C13, FALSE, D2:D13) totals the cost of the items still open.
Checkboxes are data validation underneath (Data > Data validation shows a Checkbox rule), which is why typing "yes" into one isn't the same as ticking it.