When a (typical) Linux system boots up, it first goes through an "early boot" environment that just has some basic drivers and things. The entire purpose of this environment is to find where your actual root file system is (which could theoretically be on something quite complicated, like RAID or a network file system), mount that, and then transition to the "real" system.
That error appears when something goes wrong with mounting the real file system.
When a (typical) Linux system boots up, it first goes through an "early boot" environment that just has some basic drivers and things. The entire purpose of this environment is to find where your actual root file system is (which could theoretically be on something quite complicated, like RAID or a network file system), mount that, and then transition to the "real" system.
That error appears when something goes wrong with mounting the real file system.