Here are two sentences:
The Mount St. Helens eruption produced tons of ash. The ash rained down on eastern Washington.
The word "ash" is used at the end of one sentence. "The ash" is the subject of the next sentence.
We can join the two sentences by substituting "ash" with "that" like this:
The Mount St. Helens eruption produced tons of ash that rained down on eastern Washington.
The phrase "that rained down on eastern Washington" is a relative clause.
Here is another example.
Krakatoa is a volcano in Indonesia. The volcano caused a huge tsunami when it erupted in 1883.
The word "it" stands for "the volcano in Indonesia."
We can join the two sentences by substituting "the volcano" with "that" like this:
Krakatoa is a volcano in Indonesia that caused a huge tsunami when it erupted in 1883.
The phrase "that caused a huge tsunami when it erupted in 1883" is a relative clause.
Ready to try that yourself?