This is a great question. During the third century in Roman history, there were many difficulties. In particular, there were three great hardships.
First, there was internal fighting in the form of civil war. It started with the assassination of Emperor A. Severus and after this there was nearly fifty years of constant civil war, during which time there were over twenty men claiming to be emperor. The army usually backed their own candidates and this caused great bloodshed.
Second, there was also external pressures from foreign armies. The Gauls were a constant pressure and very formidable foe, especially with internal strugglesRome was facing.
Third, there were great financial problems as well. The Roman Empire began debasing their coinage and this lead to great inflation. Diocletian came into this context and even had to issue a maximum price edict.
This significance of this time period was that it showed that the Romans were cracking under pressure. Of course, the empire would come back under Diocletian to some extent, but the aura of invincibility was gone.