"And We certainly sent Moses with Our signs," meaning: with Our proofs that indicate the validity of his prophethood, "and a clear authority," meaning an evident proof.
﴿وَلَقَد أرسلنَا مُوسَى بِآيَاتِنَا﴾ أَيْ: بِعَلامَاتِنَا الَّتِي تَدُلُّ عَلَى صِحَة نبوته ﴿وسلطانٍ مُبين﴾ حُجَّةٍ بَيِّنَةٍ.