What is a principal orbit

I am currently reading a paper about Einstein manifolds.

There is a comment where I don't know exactly the meaning of the words, namely a certain metric has a group of isometries of dimension $4$ which admits a principal orbit of dimension $3$.

What does the principal orbit describe?

Many thanks for your help!

These concepts are explained in every introduction to smooth Lie group actions on manifolds, for example chapter $$IV$$ in Bredons book "Introduction to compact transformation groups" (even though it is a standard reference I personally find this book somewhat hard to read).
To give a short overview: The isometry group $$G$$ of a complete Riemannian manifold $$(M,g)$$, or any closed subgroup of it, is a Liegroup acting on $$(M,g)$$ by isometries. The orbits $$G \ast p$$ and $$G \ast q$$ of points $$p$$ and $$q$$ are said to be of the same type if the istropy groups are conjugate, that is there exists $$g \in G$$ with $$G_p = gG_qg^{-1}$$. $$G \ast p$$ is of lower type than $$G \ast q$$ if a conjugate of $$G_q$$ is a subgroup of $$G_p$$, intuitively $$G_q$$ is smaller than $$G_p$$. It is a well known theorem, that in case $$(M,g)$$ is compact and connected there exists a maximal orbit type, that is there exists some $$q \in M$$ such that any other orbit has the same or bigger type than $$G \ast q$$. An orbit of maximal type is then called a principal orbit. There are many important structure theorems on this, e.g. the set of principal orbits is open, dense and convex in $$M$$.
Moreover any orbit $$G \ast p$$ is an embedded submanifold and hence has a dimension. In fact $$G \ast p \cong G/G_p$$. This also shows, that orbits of bigger type are essentially bigger and principal orbits are the biggest orbits.