This is the 850th most frequent Hebrew word.
שהייתה
"That was" or "which was."
In this sentence, "שהייתה" acts as a connector introducing a relative clause, describing the time as full of joy.
היא זכרה את התקופה שהייתה מלאה בשמחה.
She remembered the time that was full of joy.
Here, "שהייתה" links the subject with a state of being (in the stress).
המתח שהייתה בו השפיע על בריאותה.
The stress she was under affected her health.
In this example, "שהייתה" specifies the past presence of the car in the location being mentioned.