To be a function, each x-value (or input) can only be used once. Each x-value (or input) can only be paired with a single y-value (or output).
If any x-value (or input) is paired with more than one y-value (or output), it's not a function.
Top Left:
Not a function
"rock" and "leaf" are paired with two outputs.
Top Right:
Not a function
The one input goes to three outputs.
Bottom Left:
Function
Each x-value/input is used only once.
Bottom Right:
Not a function
The x-value of 4 is used twice and paired with two different y-values.