'Call rate' is the proportion or percentage of samples in which a confident genotype call could be made. 95% is a typical value.
In the other samples in which a call could not be made, the probes may have failed, resulting in no binding to the template DNA. It is also possible that the DNA was lesser quality in these samples, or non-existent, for whatever reason(s).
Why 95%? - it's an arbitrary cut-off. It means that the maximum missingness that we want to tolerate for any SNP is 5%.