kmer content warning in FastQC
 

I would be so grateful if someone wouldn't mind taking a look at the kmer content graph from FastQC. This is RNA-seq data after trimming and filtering based on quality scores. My questions are: Does the y-axis represent frequency? If it does why are these kmers so frequently found? What could they be? Transposons? DNA? I should mention, that there are no overrepresented sequences detected by FastQC.

Thank you in advance for your time!!!