The phrase "could be" is used to express possibility or uncertainty about something. It is similar to the phrases "might be" or "may be." For example, if you see a person with red hair, you could say "that could be my long lost brother." This means that you are not sure if the person really is your brother, but it is possible that they might be.
Here are some additional examples of how "could be" can be used:
It could rain later today.
She could become the next president.
That idea could work.
"Could be" can also be used as a polite way to ask someone for something. For example, if you want someone to pass you the salt, you could say "it could be the salt."