Filter a list down to what matters

A filter hides rows that do not match. The data is all still there, which is the difference between filtering and deleting, and the reason filtering is safe to do on somebody else's spreadsheet.

The thing to remember is that a filtered view is still a view of the whole sheet. SUM over a filtered column adds up the hidden rows too, which catches people out constantly, and is why SUMIF and SUBTOTAL exist.