The word "ancient" describes things that are thousands of years old. Some examples of "ancient" things include:

• the Egyptian pyramids
• the redwood trees in California that are over a thousand years old
• old religious stories from thousands of years ago

Because the word "ancient" has such an extreme meaning, people often use it to exaggerate about how old something is. In this example:

Look at that phone — that thing is ancient!

...the phone is only a few years old, but that seems very old because new phones are coming out so quickly. These days, people actually use "ancient" quite often to talk about technology that's old and out-of-date.