These are called histograms, a tool for evaluating distributions.
In QIIME 2, samples and features are orthogonal axes of your dataset — samples are the biological samples (or replicates, or whatever is most appropriate there), while features can represent ASVs, OTUs, Species, Metabolites, Proteins, whatever! So, the features are the “things” present in your samples.
The first plot is showing you that most of your samples only have 0-25 features (ASVs if you ran DADA2).
The second plot is telling a similar story — most of your samples only have 0-25 features present.
That is a rug plot. It is a 1D scatterplot, showing the same data as found in the histogram, sans binning.
No, that isn’t quite right — that part of the plot is saying that you have 3 samples that have between ~90,000 and 120,000 sequences (features). The total number of sequences is listed elsewhere in the same visualization.
I think I answered that above.
Please see my link at the top of my post about histograms — I think you might be confusing that with a bar chart — these are two different plots (that sort of look the same!).
On import is usually how people declare the names. Then, in your metadata, you will have a
sample-id column with the same names.