Incense symbolized prayer, as is clear from Revelation 5:8 (see also Psalm 141:2). The martyred saints under the altar were crying out to God (Revelation 6:10), and that is prayer. But every prayer from any fallen human being is unacceptable to God; there is, for example, so much selfishness in our prayers.
So incense was used in the tabernacle to make a sinner’s prayer a pleasing aroma
to God (Leviticus 2:1–2; see also Psalm 141:2). Metaphorically, the incense hid the stench of remaining sin.
3 And another angel came and stood at the altar with a golden censer, and he was given much incense to offer with the prayers of all the saints on the golden altar before the throne,