Hi- the white things are the cast-off molted skins of aphids, a minor sucking insect pest. Aphid feeding on tomato causes upward curling. You can probably see live aphids walking around on the leaves, top and bottom.

You may also see signs of biological control- brown aphid mummies that were parasitized by tiny wasps or ladybird beetle larvae and adults or syrphid fly and lacewing larvae that gobble up aphids. Aphids can also be dislodged with a water spray or insecticidal soap.
https://extension.umd.edu/hgic/topics/aphids-vegetables
Jon

Will the aphids hurt the tomatos? Is it important to remove them? Thank you.

There are many insects that prey on aphids, so unless they are causing extensive leaf curling or other damage, they are fairly harmless. Some can transmit viral plant diseases (which are incurable), but if no symptoms are present and the aphid populations low, removing them is not necessary. Populations typically decline in summer as predators come out of dormancy and start to find their colonies.

Miri