What does Acts 7:44 mean?

The "tent of witness," or tabernacle, was a large tent in a courtyard that housed the smaller items of worship, such as the lampstand, the holy bread, and the altar of incense, as well as the ark of the covenant. In front of the tent was the altar where the Israelites gave their sacrifices. When the Israelites were at Mount Sinai, shortly after their escape from Egypt, Moses would go to the top of the mountain and speak with God. God ordained that Moses would supervise the building of the tabernacle and gave him specific plans to follow (Exodus 25—27). The plans ensured the people only sacrificed to God and excluded worship of goat demons and other Egyptian gods. Moses built the tabernacle according to the plans (Exodus 38—40) and ordained his brother Aaron and Aaron's sons as the priests who performed the sacrifices and the rituals inside.

The tabernacle was a "tent of witness" or "testimony" because it provided proof God was with the Israelites. While at Mount Sinai, Moses spoke to God on Mount Sinai, but once the Israelites left, God spoke to him in the tent of meeting (Exodus 33:11). The tabernacle and its successors served as the legitimate home of the ark of the covenant and the sacrifices until Solomon built the temple, almost five hundred years later (1 Kings 6).

Stephen is laying down his defense against the Jews who claim he disrespects the temple: God didn't ask for the temple and He had no problem speaking to the priests in a tent. The building is not sacred unless God's presence is actually there.