A bluetooth speaker is a speaker to which you can stream audio over bluetooth. Although there are different types of speakers that support bluetooth streaming, a 'bluetooth speaker' means a speaker with a built-in battery.
You can use such a speaker completely wirelessly, wherever you are. Unlike Wi-Fi speakers, you are not dependent on an internet connection to stream music. That is why bluetooth speakers are often aimed at easy moving and carrying. Just think of a nice BBQ in the summer or a dinner with nice music in the back. You just take it with you wherever you like to have music.