When people make a concerted effort, they do not each pull their own way. They plan together, they keep at it, and they push in the same direction on purpose. You meet the word most in writing and in the news: a concerted campaign, concerted action, a concerted attack. It is not a lazy “we all kind of tried.” The effort is organised, and it is determined. The feel is a little formal; in everyday talk a student would more often say “we all tried together” or “we really went at it as a team.” People stretch it a little, too: one school or one company can make a “concerted effort,” meaning they organised themselves carefully, not that two countries sat down at a table. Though actually, it is not about a music concert, even though it looks like one. It sits on an older verb concert, “to plan something together,” and on the phrase in concert, “acting as one.” Musicians do still use concerted in a narrower way: concerted music is written for several players or voices working as a group, not for one person alone. That use is specialist. For most readers the “planned together, and hard” sense is the one that will turn up.