Hey Alan, I’m not sure what that instructor was speaking to specifically because rock and blues can come in so many different time signatures. Your typical blues is in 4/4 time (meaning four beats to the measure) .. so you would count it something like… 1,2,3,4 .. 2,2,3,4 .. 3,2,3,4 .. 4,2,3,4 - and then repeat. So you can see that there were four measures and four beats to a measure. Your typical blues rhythm has 12 bars (or measures) - that’s why it’s called 12 bar blues. Obviously there can be lots of twists and variations on it, but it’s all sort of based on that predictable 12 bar model.
I think the thing I’ve learned is not to get too caught up in understanding the mechanics or theory behind it, and just play it by feeling. This is where I fall short as a guitar “instructor” because I don’t understand a lot of theory - just sort of go off what my ear tells me.
I may have stated the obvious, but hopefully that will be helpful to someone.