From Ephinea PSO Wiki

Meseta icon.pngMeseta (メセタ) is the in-game currency of Phantasy Star Online Blue Burst. It can be obtained as a drop from nearly every monster and box in the game, and is often given as a reward for completing quests (with a notable exception of government quests).

Meseta can be used to buy weapons, tools, and armor (frames, barriers, and units) from the in-game shops, as well as to pay for tekking services to identify weapons. Additionally, it can be traded between players for goods and services.

Specific to Ephinea, it can also be given to Coren, the wandering tekker, to gamble for items.