What do economists mean by a "bubble"? why do many economists believe that there was a housing bubble in the united states between 2000 and 2005?

A bubble is a situation in which there is a rapid escalation of asset prices which is later followed by a contraction of the same. When there is a surge in asset prices which is unwarranted by the fundamentals of the assets that are in question and an exuberant market behavior supports it, a bubble is created. When nobody buys anymore and starts selling everything off then the bubble is deflated.

In that period, many people started buying homes with mortgages with adjustable rates. When the stocks started rising so did the prices of mortgage interest rates and people started realizing they couldn't pay back their loans and started losing homes. When the homes were taken away, there was a realization that the houses were not worth at all the price that was owed and that banks would suffer severe losses because of the bad mortgages that they gave. This led to the 2008 recession.