Modern Jive is usually danced to music with a 4/4 time signature - nearly all pop music has this time signature and is also known as common time.
There are 4 beats to bar and at the fundamental level we step on each beat. As we get more advanced we can change the footwork so long as we are aware of the underlying timing. There are usally 4 bars in a phrase.
At class, we introduce a recommended foot work in the improvers review after the beginner class:
The last beat of a previous bar is defined as an upbeat which makes the first beat of a bar a downbeat. There are 4 beats to a bar and so we have,
We take our first step on the & of the preceding bar.