WHERE keeps the rows that pass a test and drops the rest. It goes after FROM:
SELECT name, price
FROM products
WHERE price < 20;
Compare numbers with =, <> (not equal), <, >, <= and >=. Text goes in single quotes: WHERE category = 'Bath'. Text comparisons are case-sensitive, so 'bath' doesn't match 'Bath'. Double quotes mean something else in SQL (a column name), so use single quotes for values.
Dates are written as text in year-month-day order, like '2025-03-01', and compare the way you'd expect: earlier dates are smaller.
The database tests the condition on every row. Rows where it's true come back; everything else is left out. You can filter on a column without selecting it.