A very narrow slot or cut in something is a slit. You might fill your pita bread with falafel by first making a slit in its side.
A long strip of sunlight might shine through a slit in your curtains and wake you up in the morning. Or you may narrow your eyes to slits in anger when you see the mess your sister made in your room. You can also use this word as a verb, to mean "cut into," like when you slit a croissant and insert jam, or slit an envelope with an old-fashioned letter opener.