@scruff9252 – the lower grades : 1, 2, 3, 4 are walking and scrambling grades. So you’ll hear American climbers talking about “3rd class” territory meaning easy scrambling. The 5th grade is for roped climbing.
Originally the subdivisions were decimal (hence the original name Yosemite Decimal System for the grading) and went up to ten, so 5.1 – 5.10 but then people became good and you got 5.10a, 5.10b, 5.10c and 5.10d. At this point the leading climbers realised they had a potential future problem and effectively dropped the “decimal” requirement of the name and you then got 5.11(a/b/c/d) and so on.
So the “5.” part of the grade is saying “roped climbing” and the second part is the level of difficulty.