The ProBook 655 G1 has an SSD storage medium which is better than the HDD storage medium found on most notebook PCs of a similar age and spec to it. Solid State Drives (SSD) store data in semiconductors on a circuit board rather than using magnetic or optical media. Solid State Drives have no moving parts which makes them very fast and reliable.
An SSD storage medium is in fact the best storage medium amongst all notebook PCs of a comparable type and age to the ProBook 655 G1.
An HDD storage medium is the worst storage medium across similar types of laptop.