Open a new tab: Ctrl + T
Close your current tab: Ctrl + W
Open the last closed tab: Ctrl + Shift + T
Try it! Press Ctrl + T to open a new blank tab, go to your favorite website, then press Ctrl + W to close it. Now that it's closed, you realize that you actually wanted to keep that tab around, so press Ctrl + Shift + T and magically the tab you closed comes back!
There are several other helpful shortcuts but I'll leave those for another day.