The goal of Hanabi is to get all of the cards played on the stacks. Thus, we give Play Clues to each other to communicate that specific cards are playable.
Here, Alice clues red to Bob, touching a card on slot 2.
This is a message to "play this card". Thus, it must be the red 1 (because no red cards are played on the stacks yet).
After the clue, all players make a note on the card that it is a red 1.
Play Clues can be given with either a color clue or a number clue.
Here, Alice clues number 1 to Bob, touching a card on slot 3.
Similar to before, this is a message to "play this card". Thus, it must be a yellow 1 or a blue 1 (since those are the only two 1's that are not played yet).
After the clue, all players make a note on the card that it is yellow 1 or a blue 1.