This is the 318th most frequent French word.
plein
Full.
Here, 'plein' is used as an adjective meaning 'full', describing the glass being filled with water.
Le verre est plein d'eau.
The glass is full of water.
In this context, 'plein' is used colloquially to indicate a large quantity or number of people.
Il y avait plein de monde à la fête.
There were plenty of people at the party.
Here, 'plein' is used in the feminine form 'pleine' as an adjective to signify 'filled with', referring to the day being filled with surprises.