clusterProfiler, groupGO: meaning of GeneRatio
As you seem to be learning, the clusterProfiler package is a bit "special". It doesn't filter the results you're seeing for relevance, it returns everything. The GeneRatio is the number of genes in your input list associated with the given GO term (e.g., 5) and then a / and the total number of input genes. This is generally not that useful. Since many things aren't documented, you may find the source code helpful.