This verse sets the stage for a conflict between the Seleucid Empire and the Jewish people under the leadership of Jonathan Maccabeus. Let's break it down:

"And Demetrius appointed Apollonius, who was over Coelesyria...": Demetrius refers to Demetrius I Soter, a ruler of the Seleucid Empire, which controlled a vast territory in the Middle East. He appointed Apollonius as governor or some high-ranking official in charge of Coelesyria. Coelesyria was a region that included southern Syria and parts of Palestine/Judea, so it was a strategically important area.
"...and he gathered together a great army, and encamped in Jamnia...": Apollonius, acting on behalf of Demetrius, assembled a large army and set up camp in Jamnia (modern Yavne), a coastal city in Judea. This implies a military threat aimed at Jonathan and the Jewish people.
"...and sent to Jonathan the high priest, saying...": Apollonius, now with a military force at his disposal, sent a message to Jonathan, who held the position of high priest and was the leader of the Jewish people after the Maccabean Revolt. The fact that he sent a message rather than immediately attacking suggests a desire to initially engage in some sort of negotiation, intimidation, or perhaps a deceitful ploy.

In essence, the verse describes the Seleucid ruler Demetrius setting up a military confrontation with Jonathan. Apollonius is sent to Judea with a large army, encamping in Jamnia and initiating communication with Jonathan, indicating an impending clash. This situation is a direct consequence of the political and military struggles following the Maccabean revolt. The stage is being set for further actions described in the verses that follow.

The verse is also important because it highlights the ongoing power struggles and external threats faced by the Jewish people during this period. Despite achieving some level of independence, they still had to contend with the might of the Seleucid Empire and its internal power struggles.