What is the caravan?

After the US Midterm of elections 2018. The following news about the caravans:

Overview Caravan(BBC News)

https://www.bbc.com/news/world-latin-america-46207034?intlink_from_url=https://www.bbc.com/news/topics/cpz748kpzp8t/migrant-caravan&link_location=live-reporting-story

Trump seems to forget caravan.(New york Times)
https://www.nytimes.com/2018/11/13/us/politics/trump-caravan-midterms.html