A transit is when a planet or other heavenly body cross in between earth and the sun, making a shadow on the sun, basically if you have a telescope you would see the spherical shape of the planet in question move slowly across the sun. Shine on you crazy diamond!
Depending on where you live you may get to see some or all of one of the rarest astronomical events you can view. You won’t see this again until 2117. Nasa will have the whole event streaming, and if you are lucky enough to live where you can see it make sure you have protect for your eyes, staring at the sun usually doesn’t end well.
I missed Haley’s Comet in 1986 because I was a kid, and didn’t care enough to pay attention that day, now I have to wait until 2061 for the next one. I’ll be damned if i have to find a way to make it to 2117 to see something this awesome again. I’d have to make it to 137 to give it a go.