Aided acuity is the best vision you can get *with* the best optical correction being worn.
Normal good vision would be 6/6 or slightly better 6/5. even 6/4 is possible for some people.
(Consider it a fraction: "6 over 6" = 1 = normal vision, approximately)
If you have 6/7.5 and 6/15, even with best glasses you vision is somewhat below average, more so in the left eye. Though in the UK even the left eye is still within the minimum to be legal for driving.
This could just be normal for you (*someone* has to be below average: that's how they put averages together)
or there could be some reason in the eyes, such as amblyopia, or very early cataract not yet worth treating. That should have cone out in discussion of the results of your test during the appointment.
Was the result a surprise, a big change in Rx, or have your eyes always been "pretty much like that"? That could give a strong pointer but it si something to query with the person who examined your eyes, if concerned.