It's like a different language. Here are the basics:
AKA “the middle”. It’s where most of dance class is done. It’s the center of the room, but it doesn’t literally mean the center of the room. Keep away from the walls or anything you might run into (including people).

A command given by the teacher when the dancers need to find space for themselves, away from other dancers and walls. Usually an arm’s length in all directions.

Making windows or finding your window is when there are multiple lines of dancers in the center and they need to find space in-between the people in the line in front of theirs.
This makes organized lines and dancers can see themselves in the mirror without someone right in front of them.
Exercises or combinations done moving from one side of the room to the other. Dancers line up on one side of the room (usually in two lines and on the left side) and take turns doing the exercises.
Dancers will usually wait 1 to 2 eight counts before going. For beginner classes, the teacher will “count them in”–they’ll tell you when to go with a “5,6,7,8”.
For exercises, across-the-floors, or for choreography, teachers might say, “I’ll count you in” or “count yourself in”. This is the “5,6,7,8” you hear about.
It goes to the tempo of the music (or the tempo the teacher wants). It tells the dancers when to start dancing (on the next count–which would be “1”). If dancers know how to count music, they can count themselves in.
Short for “combination”, it’s a short piece of choreography (usually up to 1 minute) that you learn in center or for an across-the-floor exercise.
Combos are used to build dancers’ muscle memory library. The variety of movements strung together helps them remember dances better.
Pronounced “bar”. It’s a tool used mostly in ballet. You’ll see it attached to the walls or floor or portable ones made of wood, metal, or PVC.
It’s something for dancers to balance with while doing exercises. Hanging or sitting on the barre is a definite no-no. It’s not a playground.

“Face the back”, “go to the back”, or “start at the back” are some examples of directions using the back wall.
The mirror is the “front”, that’s your audience. The back is the opposite, away from the audience.
“Back leg” means the leg closest to the back. Same with “back arm”. “Front leg” and “front arm” are closest to the mirror.
If you know theatre directions, the front is downstage and the back is upstage.
Conclusion
These are just some of the new things you’ll hear in class–on top of all the other vocab.
Not to mention different teachers might use different terminology. It’s important to use your critical thinking skills and try to figure it out before asking questions.
When in doubt, follow what other students are doing. You’ll get it soon enough!