Hi MiddleC
No worries, you'll get there! There is a block of lessons in the Rookie Bootcamp about counting and time signatures: lessons 36-39.
But until then a quick summary: the number on the top says "how many of the time units are in one measure", the number on the bottom says "what is our time unit". See more here:
https://www.dacapoalcoda.com/42-time-signature-example
In the case of 4/2: there are 4 units in one measure, and our unit is a half-note (1/2). I.e. a measure is 2 whole notes OR 4 half-notes OR 8 quarters OR 16 eights OR any combination thereof, which is in sum equal to 4 half-notes.
What may confuse you: in playground session there is a dotted barline in the middle of the measures, when counting in 4/2. This is meant as a helper, in order so you can follow the music easier. The methodical thinking behind it is: in 4/2 you won't have as many eight- or sixteenth notes, which could be confusing (or seem formidable) for beginners. But the downside is that in 4/2 you can have looooong measures. Thus, they subdivide it with a dotted barline. The definition of a dotted barline usually is: "Subdivides long measures into shorter segments for ease of reading, usually according to natural rhythmic subdivisions.", which is exactly what's happening here: you get 4/2 measures, divided into 2 times 2/2 sub-measures with a dotted barline.
Hope this helps.
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