Good question. There is confusion in some quarters that it refers solely to the 10 commandments, but it is clear from scripture that it refers to all of God's commandments as given in the Torah (first five books of the OT). This would comprise the 10 commandments, the ceremonial aspect, and the sacrificial decrees.
This is easily observed from Luke 2:22.
And when the days of her purification according to the law of Moses were accomplished, they brought him to Jerusalem, to present him to the Lord;
The article "the" is in the original.
2:22 Καὶ ὅτε ἐπλήσθησαν αἱ ἡμέραι τοῦ καθαρισμοῦ αὐτῶν κατὰ τὸν νόμον Μωσέως ἀνήγαγον αὐτὸν εἰς Ἱεροσόλυμα παραστῆσαι τῷ κυρίῳ
In turn Luke 2:22 refers to Lev 12:2-6. Obviously this command is not one of the 10 commandments, yet it is considered part of the Law of Moses.
And as if to make this very point, Luke then writes the following regarding the law of the Lord as the same as the law of Moses.
2:23 (As it is written in the law of the Lord, Every male that openeth the womb shall be called holy to the Lord;)
Likewise we find the same Torah reference in Joshua 8:31, the first book after the books written by Moses.
As Moses the servant of the LORD commanded the children of Israel, as it is written in the book of the law of Moses, an altar of whole stones, over which no man hath lift up any iron: and they offered thereon burnt offerings unto the LORD, and sacrificed peace offerings.
In turn this command refers back to Ex 20:25 and Deu 27:6. Again neither of those commands refer to the 10 commandments.
So, the "Law of Moses" or "law of the Lord" refer to the same commandments given in Genesis, Exodus, Leviticus, Numbers, and Deuteronomy.
Some will say there are 613 commandments therein. As Peter said in Acts 15:8-11 about this issue.
And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; And put no difference between us and them, purifying their hearts by faith. ow therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear? But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they.