Introduction:
Signs of this kind can be found everywhere. Some places use this sign for confidential purposes, some for safety purposes.
Text:
Pinyin
未经许可 不得入内【wèijīng xǔkě，bù dé rù nèi】
Grammar
未经：没有经过/没有通过。 很少单用，一般都是“未经……，不得/不能 ……”
未经【wèijīng】 (no permission): unauthorized or did not receive permission. Usually used together with “不得【bù dé】/ 不能【bùnéng】”, thus, “未经【wèijīng】…, 不得【bù dé】/ 不能【bùnéng】” (no unauthorized…).
如，作者可以在文章上注明：“未经许可，不得转载”
公告中常有“未经批准，不得占用农田。”“公园内未经批准，不得使用明火。”
For example, authors can specify in his article “no unauthorized copying”.
The word also appears in notices such as, “no unauthorized occupying farmland”, or a sign in a park might say, “no unauthorized use of fire”
许可：与“允许”“批准”同义。书面语。一般都用在规定，命令中。
Xu ke (permission): synonym of “yun xu” (permission) or “pi zhun” (authorized). The word is used in written language as a provision of a mandate.
Characters: 可 得
(Source: resource.chinese.cn)