What does 防海固圉 mean?
防 ~ defence

海 ~ sea

固 ~ strong

圉 ~ prison in ancient time

roughly, "nautical defence (防海) with a strong (固) fort (圉)"

another possible translation is:

the usage of (omitted) strong (固) fort (圉) in maritime (海) defence (防).

i guess it's a summering phrase in the article "防海固圉：論明代澎湖和臺灣兵防角色之差異性"

have fun :)