I am not going into git details as net is full of git tutorials. But briefly, branch is a chain of commits, and tag points into an individual commit in the chain.
18.06 is a branch spinned off master in June 2018, and 18.06.0 and 18.06.1 have been released from it. There are already more fixes in 18.06, so 18.06.2 will have those.
But still, 18.06 is mostly June 2018 code, while current master has already five months of new development since then. And new 19.01 will be branched off master in a few weeks as a new release branch.
Openwrt branch and release strategy is illustrated in the discussion here. Hopefully it clarifies things.