Lightning and Thunder exactly happen at the same time. But you see lightning before you hear thunder because light travels more quickly than sound.
LIGHTNING: Lightning is incredibly HOT, 27,760°C (50,000°F) about 5 times hotter than the Sun’s surface. As it streaks through the sky, it heats the air so quickly that it makes a loud booming sound. This is the sound of Thunder.
THUNDER: Thunder starts in cumulonimbus clouds. They are massive clouds. The tallest can grow 18 km high, which is more than twice the height of Mt.Everest.
CONCEPT: LIGHT TRAVELS FASTER THAN SOUND