C some number (e.g. C2943672) are singleton contigs, which could not be scaffolded. scaffold some number are, obviously, scaffolded contigs. The number after them is the kmer coverage depth of the contig or scaffold, which is related to base coverage.

You could indeed filter using the kmer coverage, plot a histogram and examine its distribution. I have usually found that contigs / scaffolds with much lower coverage than the average are contaminants (not the species being sequenced), and with much higher coverage, repetitive regions. But filtering by coverage alone is not a good practice, I would combine it with other explorations of the data, as is done in blobtools.