These numbers count balanced signed graphs (without loops). A *signed graph* is a graph in which every edge has a sign, either positive or negative. It is *balanced* if every cycle has an even number of negative edges.

We will use the following lemma of Harary [Frank Harary, On the notion of balance of a signed graph, Michigan Math. J. 2 (1953/54), 143–146 (1955), Theorem 3]. I'll omit the proof, which is not difficult.

*A signed graph is balanced if and only if it is possible to color the vertices in black and white so that every positive edge joins two vertices of the same color and every negative edge joins two vertices of opposite colors.*

I'll call these colored graphs *bicolored balanced signed graphs*. It is easy to see that every connected bicolored balanced signed graph corresponds to exactly two balanced signed graphs

The number of bicolored balanced signed graphs on $[n]:=\{1,2,\dots,n\}$ is $2^{\binom{n+1}2}=2^n\cdot 2^{\binom n2}$. To see this, we construct all bicolored balanced signed graphs on $[n]$ in the following way: We start with an arbitrary graph on $[n]$. Then we color the vertices arbitrarily in black and white. Finally we make edges between vertices of the same color positive edges and we make edges between vertices of opposite colors negative edges. There are $2^{\binom n2}$ graphs on $[n]$ and $2^n$ ways of coloring the vertices of each graph in black and white, so there are $2^n\cdot 2^{\binom n2}$ bicolored balanced signed graphs.

The exponential generating function for bicolored balanced signed graphs is thus $F(x)$. So by the exponential formula, the exponential generating function for connected bicolored balanced signed graphs is $\log F(x)$. Since each connected balanced signed graph has two colorings, the exponential generating function for
connected balanced signed graphs is $\tfrac12\log F(x)$, and by the exponential formula again, the exponential generating function for balanced sign graphs is $\sqrt{F(x)}$.

The same approach shows that the polynomials in the $t_i$ count balanced signed graphs where each component with $i$ vertices is weighted $t_i$.

It is curious that the enumeration of unlabeled balanced signed graphs was accomplished by Harary and Palmer in 1967 [F. Harary and E. M. Palmer, On the number of balanced signed graphs, Bulletin of Mathematical Biophysics 29 (1967), 759–765] but the considerably easier enumeration of labeled balanced signed graphs did not appear, as far as I know, until much later
[F. Ardila, F. Castillo, Federico, and M. Henley,
The arithmetic Tutte polynomials of the classical root systems,
Int. Math. Res. Not. IMRN 2015, no. 12, 3830–3877].

There is another interpretation for the coefficients of $\sqrt{F(x)}$ which is less natural but easier to see: they count graphs on $[n]$ with loops allowed in which the least vertex in each component has a loop.