Candlemas is a Christian feast day celebrated on February 2, and it marks the Presentation of Jesus at the Temple (Luke 2:22–40). Here’s what we celebrate in simple terms:
Biblical reason to celebrate - According to Jewish law a mother went to the Temple 40 days after childbirth for purification. This is how God's firstborn son was presented to Him:
Mary and Joseph took baby Jesus to Jerusalem. There, Simeon recognizes Jesus as “a light for revelation to the Gentiles”
So that is why we light candles, because Jesus is symbolized as the Light of the World.

Over time, the Church began blessing candles on that day; holding candle processions and so we get Candle-mass → the Mass of the candles 🕯️
There is deeper symbolism, Candlemas ties together a few big Christian themes:
Light vs. darkness
Christ revealed to the world
Transition from Christmas joy toward Lent’s seriousness
It’s kind of the last echo of Christmas, which is why in some traditions Christmas decorations come down at Candlemas (not Jan 6)

There are older roots:
Candlemas absorbed elements of pre-Christian winter festivals celebrating light returning after the darkest part of the year.