The time that something is due (bordering on overdue) to be done.
For example, "It's beginning to get dark. It's high time we got started on putting up the tent". (Está mais do que na hora de começar a armar a tenda."

This is distinct from the similar 'a high time', meaning 'a happy and jolly time'.
For example, "the party went really well. A high time was had by all".

This phrase has also been used to mean 'a heated argument', but that meaning is unused and archaic now.