When I started gardening, I thought that overnight frosts would start when the air temperature goes down to 0°C or below. It wasn't long before a few dead plants proved me wrong. Given that water freezes at 0°, I don't know why we get frosts when the overnight low hits 3°, but we do.
Something else that I learnt was that tomato plants will survive in low temperatures until frost forms on the leaves, then they've had it. Aubergines however will start to wilt if they get down to about 6 degrees - they really don't like the cold.
I tend to be a bit to eager to start planting in Spring, so I get to the stage where the plants want to go outside, having outgrown their pots, but it is still too cold. Under this duress, I put out a few of the larger tomatoes plants rather early, keeping the less advanced ones indoors. I find that hiding the plants under paper hats made from folded newspaper will usually keep the frost off, whenever the forecast is for 3° or a bit lower overnight.
In the morning, the hats are either damp with dew, or frosted, so they're not good for keeping and I put them on the compost heap.